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Making a reloading manual.
- Thread starter mrawesome22
- Start date
Bullet94,
Thanks for the link. But, darn, I guess we will not get the velocities for those traces. I was thinking that Unclenick had the RSI system and produced those traces himself.
I my last post, I took the word "work" from somebody else's post and started thinking about the energy transfered to the bullet by the gas pressure, which is a function of DISTANCE times force. And, velocity can be calculated from the resulting bullet energy. But, velocity can also be calculated from acceleration over time, and acceleration comes from the force of the gas pressure on the bullets base. SO, what I wrote earlier about transforming the time scale to bullet travel distance in not necessary, but would also work if you could do it. My apology for a red-herring tangent.
BUT, either method has the problem that the pressure is not the only force on the bullet. So, if you integrate the pressure curve (using either time or distance) and use that to calculate bullet velocity, you get a result that is way too high. I did note that the RSI site says that the area under the pressure vs time curve is an "indicator" of energy delivered, not a direct measure.
One thing I noted in the RSI site is that the first figure showing pressure traces has "+" signs to indicate the time that the bullet exited the barrel, but other figures do not. The pressure trace figures do not show any distinctive shape that would allow one to pick the actual exit time by looking at the pressure. The site says that those marks are placed on that graph by "theoretical" calculations provided by versions 4.2 and later of "Pressure Trace"s software. So, I wonder how accurate they are.
I also wonder about the translation of the strain readings to pressure. It is true that a certain pressure inside the barrel will produce a certain amount of stain on the barrel's surface, all other things being held constant. BUT, temperature of the metal on the inside of the chamber wall is NOT constant. It is increased by firing the cartridge. AND, increasing the temperature of the metal on the inside without simultaneously increasing it on the outside causes the inside to try to expand while the outside tries to resist it, whicn also causes stain on the outside. So, how much of the pressure trace is due to pressure, alone?
I think all of these things (strain gauge measurments, chamber pressure measurements by copper crushers and peizoelectic meters, simulations like QuickLoad, empirical equations like Powley's) are useful for trying to understand what is happening when a cartridge is actually fired. But, none of them are complete descriptions of the real physical phenomena. They all make assumptions that are not always true.
So, if a strain guage shows a substantial increase in strain when a bullet is jammed into the rifling (compared to a bullet with a few 0.01" of free travel), I don't think we have enough solid information to discount the apparent pressure increase effect on the basis of some other calculation.
I also noted on the RSI site and one of its links that the strain guage data sometimes show "secondary pressure spikes" that may occur well out into the trace and, in some cases, may exceed the initial peak. There was also a debate over whether those were "real" or artefacts of some other phenomenon such as barrel vibrations. The site says they have been accepted as real, based in part on somebody blowing up a muzzle instead of a chamber. Real or not, the debate adds another set of phenomena to the discussion: unexpected effects on powder burning rates. Those effects may well be the reasons that some powders are not recommended for some applications, or may not be loaded to chamber pressures that reach SAAMI maximums.
SL1
Thanks for the link. But, darn, I guess we will not get the velocities for those traces. I was thinking that Unclenick had the RSI system and produced those traces himself.
I my last post, I took the word "work" from somebody else's post and started thinking about the energy transfered to the bullet by the gas pressure, which is a function of DISTANCE times force. And, velocity can be calculated from the resulting bullet energy. But, velocity can also be calculated from acceleration over time, and acceleration comes from the force of the gas pressure on the bullets base. SO, what I wrote earlier about transforming the time scale to bullet travel distance in not necessary, but would also work if you could do it. My apology for a red-herring tangent.
BUT, either method has the problem that the pressure is not the only force on the bullet. So, if you integrate the pressure curve (using either time or distance) and use that to calculate bullet velocity, you get a result that is way too high. I did note that the RSI site says that the area under the pressure vs time curve is an "indicator" of energy delivered, not a direct measure.
One thing I noted in the RSI site is that the first figure showing pressure traces has "+" signs to indicate the time that the bullet exited the barrel, but other figures do not. The pressure trace figures do not show any distinctive shape that would allow one to pick the actual exit time by looking at the pressure. The site says that those marks are placed on that graph by "theoretical" calculations provided by versions 4.2 and later of "Pressure Trace"s software. So, I wonder how accurate they are.
I also wonder about the translation of the strain readings to pressure. It is true that a certain pressure inside the barrel will produce a certain amount of stain on the barrel's surface, all other things being held constant. BUT, temperature of the metal on the inside of the chamber wall is NOT constant. It is increased by firing the cartridge. AND, increasing the temperature of the metal on the inside without simultaneously increasing it on the outside causes the inside to try to expand while the outside tries to resist it, whicn also causes stain on the outside. So, how much of the pressure trace is due to pressure, alone?
I think all of these things (strain gauge measurments, chamber pressure measurements by copper crushers and peizoelectic meters, simulations like QuickLoad, empirical equations like Powley's) are useful for trying to understand what is happening when a cartridge is actually fired. But, none of them are complete descriptions of the real physical phenomena. They all make assumptions that are not always true.
So, if a strain guage shows a substantial increase in strain when a bullet is jammed into the rifling (compared to a bullet with a few 0.01" of free travel), I don't think we have enough solid information to discount the apparent pressure increase effect on the basis of some other calculation.
I also noted on the RSI site and one of its links that the strain guage data sometimes show "secondary pressure spikes" that may occur well out into the trace and, in some cases, may exceed the initial peak. There was also a debate over whether those were "real" or artefacts of some other phenomenon such as barrel vibrations. The site says they have been accepted as real, based in part on somebody blowing up a muzzle instead of a chamber. Real or not, the debate adds another set of phenomena to the discussion: unexpected effects on powder burning rates. Those effects may well be the reasons that some powders are not recommended for some applications, or may not be loaded to chamber pressures that reach SAAMI maximums.
SL1
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Response part one
This will be a two part answer, because I've exhausted the character limit. (My friends have been telling me that for years.)
Al's response covers a lot of turf and brings up some points that will take a bit of explaining. That and the fact he is a fellow Harold Vaughn appreciator leads me to apologize for sounding a little rude in my last post, which I reread. It's no excuse to have been racing against the clock as I was, but I get cantankerous under that circumstance.
Before I start, let me add that anyone out there who has even a remotely technical bent and has not read Vaughn's book, Rifle Accuracy Facts (2nd Ed., Precision Shooting, Pub., 2000) is in for a treat when he gets hold of it.
To make an abbreviated summary of the main points in Al's post, he believes the 25% (actually 22% when I had time to run the printed numbers) pressure increase caused by a bullet touching the lands and displayed in the Pressure Trace plot I linked to is not possible. He believes this for three main reasons:
1.) Harold Vaughn measured a bullet engraving pressure at 1000 to 1500 psi, and that is not enough pressure to account for the 22% peak pressure increase on the graphs.
2.) No velocity increase appeared, so no pressure increase could have occurred.
3.) A number of people have had difficulty using the Pressure Trace instrument, so a readout from it should not be trusted.
I'll take these one-at-a-time. The middle one is a potentially dangerous misconception, so, for safety's sake, please read part two of my response (next post), if nothing else.
1.) The first point is the easiest because it is an error caused by something that plagues me increasingly as the years go by, misremembered facts. What Harold Vaughn actually measured was not the pressure needed to engrave a bullet, but the force. Vaughn (p.29, 2nd ed.), describes pushing a .270 bullet into rifling with a hydraulic press equipped with a gage that measured 1200 pounds force was needed. To find the pressure needed to supply that force, you must divide the force by the cross-sectional area of the bullet. A .270 bullet has a cross-sectional area of 0.0603 in². Dividing that into 1200 lbs equals about 20,000 lbs/in² (psi). Since he was measuring the static case, Vaughn estimated half that would be required in the kinetic case, or 600 pounds force, which, in Vaughn's own words “. . . translates to a pressure on the base of the bullet of about 10,000 psi”. In other words, the difference in start pressures between touching the lands (static case) or not touching the lands (kinetic, or sliding case) would also be about 10,000 psi.
Area = π × (0.277 in/2)², or 0.0603 in².
Pressure = 600 lbs/0.0603 in² = 9,995 lbs/in² for kinetic conditions.
Pressure = 1200 lbs/0.0603 in² = 19,990 lbs/in² for staticc conditions.
19,990 lbs/in² - 9,995 lbs/in² = 9,995 lbs/in² difference in start pressure for static and kinetic conditions.
Vaughn goes on to validate this guess with strain gage readings of pressure that, like the old HP White Laboratories experiments, show bullets off the lands starting to move at around 10,000 psi. Vaughn further suggests that while a wider diameter bullet offers more area for pressure to work on, it also has more contact surface to create friction, so these numbers wouldn't be expected to change a lot with caliber. That is not perfectly true, as I'll try to show shortly, but not way off the beam, either.
In the Pressure Trace plot I linked to, the four lower traces for bullets seated 30 mils off the lands average about 34,100 psi. This increases to an average of about 41,600 psi for the three shots measured with the bullet touching the lands. That is a difference of only 7500 psi. This is still well within the ballpark of Vaughn's own 10,000 psi expectation for start pressure increase.
But wait. There's more. What we just did was not an apples-to-apples comparison. Start pressure isn't peak pressure. An assumption that the bump in start pressure is reflected by an exactly equal increase in peak pressure is unsubstantiated. There is, actually, a bit of snowball effect. When the bullet touches the lands, the powder reaches a higher pressure before the bullet starts moving. That is due to the hesitation I mentioned in my previous post. The hesitation happens because the pressure needed to engrave the bullet in the kinetic case is already present by the time the bullet meets the throat. Unless you are using a very fast pistol powder, the 20,000 psi needed to satisfy the static case is not. So, there is a brief pause after the case neck expands off the bullet while pressure builds high enough to start the bullet forward in the static case. This allows the powder to get ahead of its burning schedule, relative to the bullet's position in the barrel. Higher pressure makes the powder burn faster. Burning rates for powders on a burning rate chart are compared at a fixed standard pressure. But like most chemical reactions, powder burning speeds change with temperature and pressure. That is is why powder goes “bang” in the gun, but “ssssst” in open air. Starting out at higher pressure means the powder mimmics a faster powder, which, for a given energy content, puts a higher portion of its pressure curve earlier in the bullet travel, and less of it further down the tube.
You can model that pressure response in QuickLOAD. QuickLOAD uses lower start pressures than 10,000 psi for several reasons. One is that it is based on modeling to match the CIP Piezo-transducer method results, which drop a couple thousand pounds compared to the SAAMI method. Another is that a bullet's actual start, if it is seated off the lands, occurs when the case neck expands away from it. QuickLOAD models little tiny increments of movement early in the burn of just millionths of an inch, way too small to be resolved by either HP White's or Harold Vaughn's oscilloscope. By the time the bullet has moved 0.030”, QuickLOAD shows pressure up in the 10,000 psi range, which is something the old instrumentation can discern. The bottom line is, the lower start pressures employed in the program return correct burning curves for the powder during bullet acceleration. Nonetheless, you can use the program to try increasing start pressure to see the effect on specific powders. For H322 in the 6 mm PPC with a 90 grain bullet, the increase in start pressure needed to get that 7,500 psi Pressure Trace measured increase is about 5,700 lbs. Less than Vaughn's 10,000 psi, but still not off by an order of magnitude or anything dramatic. The peak pressure for that powder shows an increase of about 1.3 times times the added start pressure. If you use Vaughn's 10,000 psi kinetic start pressure and increase it by 5,700 lbs., you have a static-to-kinetic coefficient of friction difference of 1:1.57 instead of 1:2. Not too far off what I originally posted from a table, even though that table was for pure copper on mild steel, and not jacket alloy on barrel steel hardened to RC 30, which would have a greater ratio. But again, it's not too far off if you believe the Pressure Trace graph. I will explain, further along, why you should believe it in this instance. Now, however, you are looking at an apparent data contradiction to Vaughn's results. I don't think it is actually very hard to explain by the difference in diameter expansion under engraving pressure. Vaughn's numbers include not just friction, but also the metal swaging forces and these change with the ratio of jacket thickness to total bullet diameter, as will bullet diameter expansion under pressure. There are more forces at work that may be described, but they won't change the bottom line much.
It may seem I have taken the discussion a bit adrift. The point really isn't whose friction coefficient tables apply. I just wanted to remind the gentle reader that guns and chamberings are individuals when it comes to maximum loads, as well as are accuracy and response to different powder and bullet combinations. It is best to be wary of generalizations about what will happen to peak pressures as a result of changes you make, and not to assume that something like putting the bullet out to touch the lands is automatically a safe practice with your equipment because it turned out OK with someone else's. Just remember to follow the general purpose admonition to knock your loads down 10% and work up again whenever you make a change in any component or loading practice.
If you are running a gun near its individual maximum pressure, it is often already a good bit above the SAAMI standard maximum, so letting a bullet touch the lands to give you another 5, 10 or 15 thousand psi or so, depending on the powder, is still enough to move a hot load up into proof pressure range, and that can't be good for the gun to have a steady diet of those. It won't give best accuracy anyway.
I don't think anything I said in my prior post about the pressure rise from letting a bullet touch the lands has substantially been disputed. To repeat myself, if you make a change, back loads off and work up again. The caution can't hurt your wallet as badly as overpressure can hurt your face, so why not go that extra mile? Safety first.
Middleton Tompkins, who has more Long Range gold medals than anyone living or dead, AFAIK, does all the loading for his whole family of national and international champions. He told a class I attended that the loads he prepares have their bullets seated way out and the case necks so loose he can press the bullets in with his fingers. The throat of the barrel is allowed to finish seating his bullets when he closes the bolt. The only drawback to this system occurs when a cease fire is called requiring unloading a live round. In that case he can be almost certain the bullet will stick in the throat. So, the muzzle has to be tipped up and the bolt opened slowly so he can catch the case to keep powder from spilling into the action. He then must use a cleaning rod to knock the bullet out before loading again.
So, creating a load to touch the lands is not inherently a bad thing. You just have to work the load up for it and be careful how you use it.
This will be a two part answer, because I've exhausted the character limit. (My friends have been telling me that for years.)
Al's response covers a lot of turf and brings up some points that will take a bit of explaining. That and the fact he is a fellow Harold Vaughn appreciator leads me to apologize for sounding a little rude in my last post, which I reread. It's no excuse to have been racing against the clock as I was, but I get cantankerous under that circumstance.
Before I start, let me add that anyone out there who has even a remotely technical bent and has not read Vaughn's book, Rifle Accuracy Facts (2nd Ed., Precision Shooting, Pub., 2000) is in for a treat when he gets hold of it.
To make an abbreviated summary of the main points in Al's post, he believes the 25% (actually 22% when I had time to run the printed numbers) pressure increase caused by a bullet touching the lands and displayed in the Pressure Trace plot I linked to is not possible. He believes this for three main reasons:
1.) Harold Vaughn measured a bullet engraving pressure at 1000 to 1500 psi, and that is not enough pressure to account for the 22% peak pressure increase on the graphs.
2.) No velocity increase appeared, so no pressure increase could have occurred.
3.) A number of people have had difficulty using the Pressure Trace instrument, so a readout from it should not be trusted.
I'll take these one-at-a-time. The middle one is a potentially dangerous misconception, so, for safety's sake, please read part two of my response (next post), if nothing else.
1.) The first point is the easiest because it is an error caused by something that plagues me increasingly as the years go by, misremembered facts. What Harold Vaughn actually measured was not the pressure needed to engrave a bullet, but the force. Vaughn (p.29, 2nd ed.), describes pushing a .270 bullet into rifling with a hydraulic press equipped with a gage that measured 1200 pounds force was needed. To find the pressure needed to supply that force, you must divide the force by the cross-sectional area of the bullet. A .270 bullet has a cross-sectional area of 0.0603 in². Dividing that into 1200 lbs equals about 20,000 lbs/in² (psi). Since he was measuring the static case, Vaughn estimated half that would be required in the kinetic case, or 600 pounds force, which, in Vaughn's own words “. . . translates to a pressure on the base of the bullet of about 10,000 psi”. In other words, the difference in start pressures between touching the lands (static case) or not touching the lands (kinetic, or sliding case) would also be about 10,000 psi.
Area = π × (0.277 in/2)², or 0.0603 in².
Pressure = 600 lbs/0.0603 in² = 9,995 lbs/in² for kinetic conditions.
Pressure = 1200 lbs/0.0603 in² = 19,990 lbs/in² for staticc conditions.
19,990 lbs/in² - 9,995 lbs/in² = 9,995 lbs/in² difference in start pressure for static and kinetic conditions.
Vaughn goes on to validate this guess with strain gage readings of pressure that, like the old HP White Laboratories experiments, show bullets off the lands starting to move at around 10,000 psi. Vaughn further suggests that while a wider diameter bullet offers more area for pressure to work on, it also has more contact surface to create friction, so these numbers wouldn't be expected to change a lot with caliber. That is not perfectly true, as I'll try to show shortly, but not way off the beam, either.
In the Pressure Trace plot I linked to, the four lower traces for bullets seated 30 mils off the lands average about 34,100 psi. This increases to an average of about 41,600 psi for the three shots measured with the bullet touching the lands. That is a difference of only 7500 psi. This is still well within the ballpark of Vaughn's own 10,000 psi expectation for start pressure increase.
But wait. There's more. What we just did was not an apples-to-apples comparison. Start pressure isn't peak pressure. An assumption that the bump in start pressure is reflected by an exactly equal increase in peak pressure is unsubstantiated. There is, actually, a bit of snowball effect. When the bullet touches the lands, the powder reaches a higher pressure before the bullet starts moving. That is due to the hesitation I mentioned in my previous post. The hesitation happens because the pressure needed to engrave the bullet in the kinetic case is already present by the time the bullet meets the throat. Unless you are using a very fast pistol powder, the 20,000 psi needed to satisfy the static case is not. So, there is a brief pause after the case neck expands off the bullet while pressure builds high enough to start the bullet forward in the static case. This allows the powder to get ahead of its burning schedule, relative to the bullet's position in the barrel. Higher pressure makes the powder burn faster. Burning rates for powders on a burning rate chart are compared at a fixed standard pressure. But like most chemical reactions, powder burning speeds change with temperature and pressure. That is is why powder goes “bang” in the gun, but “ssssst” in open air. Starting out at higher pressure means the powder mimmics a faster powder, which, for a given energy content, puts a higher portion of its pressure curve earlier in the bullet travel, and less of it further down the tube.
You can model that pressure response in QuickLOAD. QuickLOAD uses lower start pressures than 10,000 psi for several reasons. One is that it is based on modeling to match the CIP Piezo-transducer method results, which drop a couple thousand pounds compared to the SAAMI method. Another is that a bullet's actual start, if it is seated off the lands, occurs when the case neck expands away from it. QuickLOAD models little tiny increments of movement early in the burn of just millionths of an inch, way too small to be resolved by either HP White's or Harold Vaughn's oscilloscope. By the time the bullet has moved 0.030”, QuickLOAD shows pressure up in the 10,000 psi range, which is something the old instrumentation can discern. The bottom line is, the lower start pressures employed in the program return correct burning curves for the powder during bullet acceleration. Nonetheless, you can use the program to try increasing start pressure to see the effect on specific powders. For H322 in the 6 mm PPC with a 90 grain bullet, the increase in start pressure needed to get that 7,500 psi Pressure Trace measured increase is about 5,700 lbs. Less than Vaughn's 10,000 psi, but still not off by an order of magnitude or anything dramatic. The peak pressure for that powder shows an increase of about 1.3 times times the added start pressure. If you use Vaughn's 10,000 psi kinetic start pressure and increase it by 5,700 lbs., you have a static-to-kinetic coefficient of friction difference of 1:1.57 instead of 1:2. Not too far off what I originally posted from a table, even though that table was for pure copper on mild steel, and not jacket alloy on barrel steel hardened to RC 30, which would have a greater ratio. But again, it's not too far off if you believe the Pressure Trace graph. I will explain, further along, why you should believe it in this instance. Now, however, you are looking at an apparent data contradiction to Vaughn's results. I don't think it is actually very hard to explain by the difference in diameter expansion under engraving pressure. Vaughn's numbers include not just friction, but also the metal swaging forces and these change with the ratio of jacket thickness to total bullet diameter, as will bullet diameter expansion under pressure. There are more forces at work that may be described, but they won't change the bottom line much.
It may seem I have taken the discussion a bit adrift. The point really isn't whose friction coefficient tables apply. I just wanted to remind the gentle reader that guns and chamberings are individuals when it comes to maximum loads, as well as are accuracy and response to different powder and bullet combinations. It is best to be wary of generalizations about what will happen to peak pressures as a result of changes you make, and not to assume that something like putting the bullet out to touch the lands is automatically a safe practice with your equipment because it turned out OK with someone else's. Just remember to follow the general purpose admonition to knock your loads down 10% and work up again whenever you make a change in any component or loading practice.
If you are running a gun near its individual maximum pressure, it is often already a good bit above the SAAMI standard maximum, so letting a bullet touch the lands to give you another 5, 10 or 15 thousand psi or so, depending on the powder, is still enough to move a hot load up into proof pressure range, and that can't be good for the gun to have a steady diet of those. It won't give best accuracy anyway.
I don't think anything I said in my prior post about the pressure rise from letting a bullet touch the lands has substantially been disputed. To repeat myself, if you make a change, back loads off and work up again. The caution can't hurt your wallet as badly as overpressure can hurt your face, so why not go that extra mile? Safety first.
Middleton Tompkins, who has more Long Range gold medals than anyone living or dead, AFAIK, does all the loading for his whole family of national and international champions. He told a class I attended that the loads he prepares have their bullets seated way out and the case necks so loose he can press the bullets in with his fingers. The throat of the barrel is allowed to finish seating his bullets when he closes the bolt. The only drawback to this system occurs when a cease fire is called requiring unloading a live round. In that case he can be almost certain the bullet will stick in the throat. So, the muzzle has to be tipped up and the bolt opened slowly so he can catch the case to keep powder from spilling into the action. He then must use a cleaning rod to knock the bullet out before loading again.
So, creating a load to touch the lands is not inherently a bad thing. You just have to work the load up for it and be careful how you use it.
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Response part two
Onward and outward:
2.) Now let's clear up this misconception: Higher pressure is always disproved if increased bullet velocity is not observed.
This is simply not so in the maximum pressure range. In fact, lack of increased velocity even with increased powder charge or even loss of velocity with increased powder charge and pressure, are standard high pressure signs watched for in load development. It is the one pressure sign a chronograph can give you. For an example, see Mrawesome22's results in post #15 above, where his increase in charge that began producing hard bolt lift suffered an 80 fps loss in velocity over the next lower charge he tried. Also, see Bullet94's results in post #14, his test with bullets touching the lands produced a small and possibly insignificant loss of 20 fps over those not touching the lands. At least we can say it clearly didn't gain anything. This is likely an early indicator of encroaching on the maximum pressure range.
Well, gee! Increasing powder charge increased both pressure and velocity at lower levels. What the heck is the difference in the maximum pressure range? The short answer is that under higher pressure the gun metal exhibits greater strain and the powder burns hotter and the bore is stretched and distorted more. All these conditions increase energy loss as heat. And here's the rub: The powder charge only has so much energy to give, and what isn't going into the bullet is going somewhere else. 'Somewhere else' gets a bigger percentage of the energy when you cross over the gun's maximum working pressure range and the chamber starts to expand excessively and the barrel to stretch more, and what is left is less for the bullet. A net decrease in ballistic efficiency results. You are then in a pressure range that accelerates metal fatigue, too. Military guns typically are run at about 40% of yield for the steel or less, and after many thousands of rounds the actions remain serviceable over a century later. Modern guns running at up to 60% of yield won't likely provide that kind of service history, but only future generations will live to find out for sure. Note that accuracy also diminishes as you go up into the more extreme metal stretching pressure region. It creates some bad vibrational irregularities, so there is usually no good reason to shoot up there anyway.
3.) So, now we get to the accuracy of the measured pressure curves I linked to. I do own a Pressure Trace instrument, but have never done the touching bullet experiment without reducing powder charge first, so I borrowed the posted graph for expedience.
I've looked closely at those RSI plots. I'm not sure why RSI referred to the load as being popular? The 90 grain bullet and H322 powder are a popular pairing in 6 mm PPC, but the loads themselves are usually a good bit warmer, as Al said. This is wimpy, but that might be why it was selected to run an experiment with the bullet out touching the lands? The experimenter wanted insurance the resulting spike was not going to stress his gun, but overdid the compensation. That's OK. Always err on the side of safety.
The plots appear to be good readings and look accurate. Here's why I say that: First, if you inspect them closely, you will see they have the right shape. The curves that hit the higher peak fall below the curves from the lower peak out near the 1 millisecond point. The graphs also drop very low at the end, indicating a mild load. Pressure Trace or other strain gage system graphs don't show a drop at the moment the bullet exits the muzzle because it takes time for the muzzle pressure drop to work its way back to the chamber where the gage is. I think that confuses a lot of people using the instrument who don't realize there is a delay between what happens at the base of the bullet and what happens at the chamber.
Secondly, if I use QuickLOAD to run a 6 mm PPC with a 90 grain bullet and an H322 charge that produces a 34,100 psi pressure, the chamber pressure at 1 millisecond matches the pressure in the plots of the low peaks pretty closely, being at 12,000 psi. If I increase the load to produce about 67,000 psi, the one millisecond pressure drops to about 9,000 PSI. The bullet is going faster with the hotter load, and puts about 8 more inches of barrel between itself and the breech at the one millisecond point than the 34,100 psi loads do. That increased volume is disproportionate to the added powder quantity, so its pressure drops a little more. It's not a very precise way to analyze the situation, but it is another indicator that the plots are fairly accurate. If they were somehow chopped by a missing offset quantity, the ratio of the peaks to the 1 ms value would be far greater.
Bottom line, the plots match calculated performance well and have every sign of representing the low pressure 34,100 psi load. I think it is pretty believable. The trouble people have had with the Pressure Trace system needs to be addressed, though, because it is a useful instrument if you employ it correctly.
Harold Vaughn used strain gages for measuring pressure, as does the Oehler ballistic lab. There is nothing wrong with the principle, but there are some pratfalls to be avoided. I have a lengthy background in instrumentation design and was lucky enough to have an engineer who did landing gear stress analysis for military aircraft teach me how to apply strain gages years ago. There is technique to it. Not difficult, but you can do it wrong, especially in the beginning. You need to know the glue line needs to be thin and free of bubbles and gaps, so roll it on with your thumb, then keep pressure constant for twice as long as you think you need to, so you don't get premature creep by pre-stressing a still-soft glue layer. The wrong adhesives will make too thick a glue line and that adds hysteresis and creep error which can cause significant problems with consistent reading and with installation durability.
RSI's (Pressure Trace maker) Jim Ristow told me the biggest problem he sees is that people don't realize the gage has an orientation, and don't read the instructions concerning this. The gage is a micro-etched conductive foil, usually nickel alloy, that forms a resistor. The pattern of the foil is back and forth foil parallel lines that resemble the path a farmer might make plowing a long field. The turns are made at the narrow ends so the number of turns is minimized. The long runs are in the direction the gage measures strain in. Since the Pressure Trace is calibrated to measure the strain resulting from hoop stress, the gage has to be oriented to wrap around the barrel over the chamber, with long foil runs perpendicular to the barrel axis. It cannot be laid down lengthwise and parallel to the bore axis. Even though it would fit more easily on guns that way, you can't get a useful measurement from that.
One of my fellow moderators at The Shooter's Forum owns a Pressure Trace, too. At one point he got so mad at his he actually tossed it in a trash heap for burning, though thought the better of it after chilling out for a bit. The inconsistent results Al mentioned were driving him nuts. After he sat down and thought it all through, though, he went back to the beginning, read the manual, and now has his instrument working very well and is very glad he kept it. His main secret was to go to an industrial Loctite supplier (Fastenal, I think) and get the Loctite 401 cyanoacrylic adhesive mentioned in the Pressure Trace manual. It really has no peer among cyanoacrylics in this application. The use of the cyanoacrylic glue makes these gages applicable without removing gun finish and allows them to be later removed with acetone. After awhile the glue bond deteriorates. You can minimize deterioration by carefully cooling the barrel between rounds. Barrel heating is a major cause of glue creep, which spoils calibration.
Permanent use is best achieved using the traditional epoxies made specifically for strain gage application, such as Micro-measurements' (now owned by Vishay) M-bond 610, but you need the metal cleared of finish and prepped for it. It also needs to cure for a week for best strength. The best prep method I've found is to use a Badger mini-abrasive blaster and something in the range of 320 grit aluminum oxide blasting medium, followed by no-residue canned degreasing solvent to blow off any trace of oil or water that was in the compressed air source used with the blaster. You need pressure applied during the whole glue setting period. This is done by rolling the gage on with a silicone rubber or a Teflon pad, then maintaining pressure on that pad with rubber bands. It's a nuisance, but doable and works pretty indefinitely if you don't let the installation get hot. Temperature changes can screw up epoxy glue bonds, too.
I hope all this helps sort out apparent discrepancies? I may put together a tip sheet sometime on how to cut down on noise and other issues in using the Pressure Trace. At the risk of sounding like a broken record, I'll again state that when you change a component or loading practice, back off and work up. Stay safe, or your wife won't let you go to the range anymore.
Nick
Onward and outward:
2.) Now let's clear up this misconception: Higher pressure is always disproved if increased bullet velocity is not observed.
This is simply not so in the maximum pressure range. In fact, lack of increased velocity even with increased powder charge or even loss of velocity with increased powder charge and pressure, are standard high pressure signs watched for in load development. It is the one pressure sign a chronograph can give you. For an example, see Mrawesome22's results in post #15 above, where his increase in charge that began producing hard bolt lift suffered an 80 fps loss in velocity over the next lower charge he tried. Also, see Bullet94's results in post #14, his test with bullets touching the lands produced a small and possibly insignificant loss of 20 fps over those not touching the lands. At least we can say it clearly didn't gain anything. This is likely an early indicator of encroaching on the maximum pressure range.
Well, gee! Increasing powder charge increased both pressure and velocity at lower levels. What the heck is the difference in the maximum pressure range? The short answer is that under higher pressure the gun metal exhibits greater strain and the powder burns hotter and the bore is stretched and distorted more. All these conditions increase energy loss as heat. And here's the rub: The powder charge only has so much energy to give, and what isn't going into the bullet is going somewhere else. 'Somewhere else' gets a bigger percentage of the energy when you cross over the gun's maximum working pressure range and the chamber starts to expand excessively and the barrel to stretch more, and what is left is less for the bullet. A net decrease in ballistic efficiency results. You are then in a pressure range that accelerates metal fatigue, too. Military guns typically are run at about 40% of yield for the steel or less, and after many thousands of rounds the actions remain serviceable over a century later. Modern guns running at up to 60% of yield won't likely provide that kind of service history, but only future generations will live to find out for sure. Note that accuracy also diminishes as you go up into the more extreme metal stretching pressure region. It creates some bad vibrational irregularities, so there is usually no good reason to shoot up there anyway.
3.) So, now we get to the accuracy of the measured pressure curves I linked to. I do own a Pressure Trace instrument, but have never done the touching bullet experiment without reducing powder charge first, so I borrowed the posted graph for expedience.
I've looked closely at those RSI plots. I'm not sure why RSI referred to the load as being popular? The 90 grain bullet and H322 powder are a popular pairing in 6 mm PPC, but the loads themselves are usually a good bit warmer, as Al said. This is wimpy, but that might be why it was selected to run an experiment with the bullet out touching the lands? The experimenter wanted insurance the resulting spike was not going to stress his gun, but overdid the compensation. That's OK. Always err on the side of safety.
The plots appear to be good readings and look accurate. Here's why I say that: First, if you inspect them closely, you will see they have the right shape. The curves that hit the higher peak fall below the curves from the lower peak out near the 1 millisecond point. The graphs also drop very low at the end, indicating a mild load. Pressure Trace or other strain gage system graphs don't show a drop at the moment the bullet exits the muzzle because it takes time for the muzzle pressure drop to work its way back to the chamber where the gage is. I think that confuses a lot of people using the instrument who don't realize there is a delay between what happens at the base of the bullet and what happens at the chamber.
Secondly, if I use QuickLOAD to run a 6 mm PPC with a 90 grain bullet and an H322 charge that produces a 34,100 psi pressure, the chamber pressure at 1 millisecond matches the pressure in the plots of the low peaks pretty closely, being at 12,000 psi. If I increase the load to produce about 67,000 psi, the one millisecond pressure drops to about 9,000 PSI. The bullet is going faster with the hotter load, and puts about 8 more inches of barrel between itself and the breech at the one millisecond point than the 34,100 psi loads do. That increased volume is disproportionate to the added powder quantity, so its pressure drops a little more. It's not a very precise way to analyze the situation, but it is another indicator that the plots are fairly accurate. If they were somehow chopped by a missing offset quantity, the ratio of the peaks to the 1 ms value would be far greater.
Bottom line, the plots match calculated performance well and have every sign of representing the low pressure 34,100 psi load. I think it is pretty believable. The trouble people have had with the Pressure Trace system needs to be addressed, though, because it is a useful instrument if you employ it correctly.
Harold Vaughn used strain gages for measuring pressure, as does the Oehler ballistic lab. There is nothing wrong with the principle, but there are some pratfalls to be avoided. I have a lengthy background in instrumentation design and was lucky enough to have an engineer who did landing gear stress analysis for military aircraft teach me how to apply strain gages years ago. There is technique to it. Not difficult, but you can do it wrong, especially in the beginning. You need to know the glue line needs to be thin and free of bubbles and gaps, so roll it on with your thumb, then keep pressure constant for twice as long as you think you need to, so you don't get premature creep by pre-stressing a still-soft glue layer. The wrong adhesives will make too thick a glue line and that adds hysteresis and creep error which can cause significant problems with consistent reading and with installation durability.
RSI's (Pressure Trace maker) Jim Ristow told me the biggest problem he sees is that people don't realize the gage has an orientation, and don't read the instructions concerning this. The gage is a micro-etched conductive foil, usually nickel alloy, that forms a resistor. The pattern of the foil is back and forth foil parallel lines that resemble the path a farmer might make plowing a long field. The turns are made at the narrow ends so the number of turns is minimized. The long runs are in the direction the gage measures strain in. Since the Pressure Trace is calibrated to measure the strain resulting from hoop stress, the gage has to be oriented to wrap around the barrel over the chamber, with long foil runs perpendicular to the barrel axis. It cannot be laid down lengthwise and parallel to the bore axis. Even though it would fit more easily on guns that way, you can't get a useful measurement from that.
One of my fellow moderators at The Shooter's Forum owns a Pressure Trace, too. At one point he got so mad at his he actually tossed it in a trash heap for burning, though thought the better of it after chilling out for a bit. The inconsistent results Al mentioned were driving him nuts. After he sat down and thought it all through, though, he went back to the beginning, read the manual, and now has his instrument working very well and is very glad he kept it. His main secret was to go to an industrial Loctite supplier (Fastenal, I think) and get the Loctite 401 cyanoacrylic adhesive mentioned in the Pressure Trace manual. It really has no peer among cyanoacrylics in this application. The use of the cyanoacrylic glue makes these gages applicable without removing gun finish and allows them to be later removed with acetone. After awhile the glue bond deteriorates. You can minimize deterioration by carefully cooling the barrel between rounds. Barrel heating is a major cause of glue creep, which spoils calibration.
Permanent use is best achieved using the traditional epoxies made specifically for strain gage application, such as Micro-measurements' (now owned by Vishay) M-bond 610, but you need the metal cleared of finish and prepped for it. It also needs to cure for a week for best strength. The best prep method I've found is to use a Badger mini-abrasive blaster and something in the range of 320 grit aluminum oxide blasting medium, followed by no-residue canned degreasing solvent to blow off any trace of oil or water that was in the compressed air source used with the blaster. You need pressure applied during the whole glue setting period. This is done by rolling the gage on with a silicone rubber or a Teflon pad, then maintaining pressure on that pad with rubber bands. It's a nuisance, but doable and works pretty indefinitely if you don't let the installation get hot. Temperature changes can screw up epoxy glue bonds, too.
I hope all this helps sort out apparent discrepancies? I may put together a tip sheet sometime on how to cut down on noise and other issues in using the Pressure Trace. At the risk of sounding like a broken record, I'll again state that when you change a component or loading practice, back off and work up. Stay safe, or your wife won't let you go to the range anymore.
Nick
Last edited:
Unclenick
Thank you for your response. I copied it to here -
http://www.benchrest.com/forums/showthread.php?t=27109&page=3
Thank you for your response. I copied it to here -
http://www.benchrest.com/forums/showthread.php?t=27109&page=3
Al’s response -
I'm not sure how to "answer" all of this as it's getting a little muddled
But that's never kept me from trying 
I'll start with Nicks interpretation of my answers........
1.) Harold Vaughn measured a bullet engraving pressure at 1000 to 1500 psi, and that is not enough force to account for the 22% peak pressure increase on the graphs.
2.) No velocity increase appeared, so no pressure increase could have occurred.
3.) A number of people have had difficulty using the Pressure Trace instrument, so a readout from it should not be trusted.
In fact NONE of these assertions are mine.... I assume this to be a case of misunderstanding and not purposeful obfuscation of he issues.
#1--- Harold Vaughn did NOT "measure a bullet engraving pressure at 1000 to 1500 psi", I noted this as the total conceivable DIFFERENCE between touching and 30thou out of the lands. Bullet engraving pressure is in the neighborhood of 10,000psi, this from Vaughn, Rinker and Howell. Furthermore confusing is the fact that Nick goes on to restate WITH DETAIL and corroboration from Vaughn EXACTLY what I was saying........??? ......I'm not sure what to make of it.......
#2-- "No velocity increase appeared, so no pressure increase could have occurred." Again, simply a misrepresentation. I make many references to pressure and velocity which may all be encapsulated by this phrase from my post, "Pressure/velocity are essentially linear". "Essentially linear" is exactly what I meant to say, with "essentially" acting as a modifier to "linear". Velocity = pressure x area x time x (gravitational constant/mass) I'm not sure why G is thrown into the mix as it seems normally to be acting at right angles to the involved forces.......you can eliminate gravity and still come up with a valid result. IMO ignoring G is LESS a problem than ignoring non-linear combustion. (again, I'm happy to be SHOWN to be wrong )
#3-- Regarding RSI data, I have no opinion regarding "people having difficulty" with RSI equipment. I have difficulty with the conclusions drawn, mostly conclusions stated as fact by the folks at RSI.
Now regarding all of the fluff about those loads which exhibit pressure increase WITHOUT a commensurate velocity increase........ Again we're getting all mixed up here. The references (mainly anecdotal) to the "plateau" which often occurs when the reloader ventures above max loading capacity are used to somehow "support?" the argument???...... the referenced, while existent, is irrelevant to this discussion. The plateau is simply the result of factors colluding to change the burn rate of the powder. Nick alludes to all of this in his explanation but doesn't apply it to THIS discussion which is quite simply stated as, "CAN letting a bullet inadvertently touch the lands cause a dangerous pressure event".......... My answer is "NO."
To agree with a lot of what Nick is saying, "can events collaborate to CHANGE THE BURN RATE of the powder charge and cause non-linear progression of Pressure/Velocity?" Absolutely YES..............but simply touching the lands ain't good for a 20-25% increase in chamber pressure. And IF true chamber pressure were to increase 20-25% then there WOULD be an accompanant velocity spike.
Some other random quotes from Nick which I disagree with.
---"The short answer is that under higher pressure the gun metal exhibits greater strain and the powder burns hotter and the bore is stretched and distorted more." . . . . . . .Yeahh, SO??? We're looking at a total difference, energy conversion, in the area of 1-2% and this difference is incremental and linear. The "bore stretch" one is REALLY a stretch! since peak pressure occurs in the first inches of the bore.....and pressure is dropping from there on...... The real answer is that the burning pressure curve is distorted to become slightly less efficient, yes it DOES waste energy in heat and excessive vibration but not from a volumetric change in the rifle. We have a TRANSFER disconnect, not a distortion problem. Distortion is a symptom. This is really no different than the energy transfer of a bat to a ball, a bow to an arrow or a throwing arm to a rock.
Statements like this one just bug me....... Nick quotes Vaughn in ref to engraving force changing with caliber saying "That is not perfectly true, as I'll try to show shortly, but not way off the beam, either." WHY restate Vaughn in a attempt to steal his thunder? The REASON that Vaughn doesn't go on to detail the differences is that it's NOT RELEVANT! It's certainly "true enough" or accurate enough to make the point!
Or this ............ "If you are running a gun near its individual maximum pressure, it is often already a good bit above the SAAMI standard maximum, so letting a bullet touch the lands to give you another 5, 10 or 15 thousand psi or so, depending on the powder, is still enough to move a hot load up into proof pressure range, and that can't be good for the gun to have a steady diet of those. It won't give best accuracy anyway." ............... WHO SEZ it "won't give best accuracy anyways?" ALL competitive Bench Rest shooters work in this range and it's the ONLY way to be competitive and furthermore these actions have taken many times more "abusive pressure levels" in their short lives than any other type of rifle will take over time. "Time" is relative. The implication here is that firing these hot loads "over time" will incrementally damage the action. Simply not true. (BTW, using Mid Tompkins as a reference shows nothing but misunderstanding of the question. Middleton has neither the desire nor the capacity to achieve truly high-level intrinsic accuracy. You might as well use GD Tubb...... or Carlos Hathcock.......or Chuck Mawhinney)
And 90gr bullets "common for PPC"? and loaded to 34,000psi???? AGAIN simply no frame of reference. This is BRC for crying out loud. NO other discipline has the same level of intrinsic accuracy nor do other shooters run at the pressures that PPC/BR and now the 6.5X47L case run. ROUTINELY running pressures that are often touted as "action-destroying". BTW, H322 and a 90gr bullet is almost certainly a recipe for disaster! 322 is 'wayy too fast for that hunormous bullet. Not really relevant, but lest anyone be misled by casual verbiage.........and the level of ignorance that led RSI to use such a mismatch???? odd
Seems like Nick views the firing event sequence essentially as I do, no quarrels there..
We surely do agree re the safety aspects of all this. Be careful out there 
I still maintain that there's a disconnect somewhere betwixt the data and the interp........work needs to be done before certain things are stated as "fact".
________________________________________________________________
Al posted again -
This post has been bugging me so I just had to come back and reread it all the way thru.........
Regarding my posts and a person from another forum named "Nick" there has been kindofa' long distance third person back-n-forth regarding the mechanics of bullet startup and the use of RSI Pressure Trace equipment.
Now this "Nick" seems a knowledgeable guy and I consider myself to be fairly well-versed on the subjects presented yet we seem to have a disagreement. Discussion GOOD, disagreement, CONFUSING......... I've been confused by several of Nick's contentions.
I MAY have found one of our disconnects.
Nick uses this quote >>>> "In other words, the difference in start pressures between touching the lands (static case) or not touching the lands (kinetic, or sliding case) would also be about 10,000 psi." <<<<< as a way of restating some of the things that Harold Vaughn talks about in his book "Rifle Accuracy Facts".
The quote is taken from this paragraph here >>>>> 1.) "The first point is the easiest because it is an error caused by something that plagues me increasingly as the years go by, misremembered facts. What Harold Vaughn actually measured was not the pressure needed to engrave a bullet, but the force. Vaughn (p.29, 2nd ed.), describes pushing a .270 bullet into rifling with a hydraulic press equipped with a gage that measured 1200 pounds force was needed. To find the pressure needed to supply that force, you must divide the force by the cross-sectional area of the bullet. A .270 bullet has a cross-sectional area of 0.0603 in². Dividing that into 1200 lbs equals about 20,000 lbs/in² (psi). Since he was measuring the static case, Vaughn estimated half that would be required in the kinetic case, or 600 pounds force, which, in Vaughn's own words “. . . translates to a pressure on the base of the bullet of about 10,000 psi”. In other words, the difference in start pressures between touching the lands (static case) or not touching the lands (kinetic, or sliding case) would also be about 10,000 psi." <<<<<
HERE I think is where Nick and I really part ways, HERE is where I believe Nick MIS-INTERPRETS what Harold is saying and is led down a path of confusion.
Nick takes this to mean that the difference between touching and not touching is 10,000psi.
WRONG! This isn't what Harold's saying A'tall.......
THIS INTERPRETATION is flawed.............. the entire CONCEPT is flawed .......... I missed this in the first go-'round because I just can't always find another person's frame of reference. The problem is mine, I should have read it over until I realized what Nick was trying to say.
What Harold Vaughn is saying is that it takes 20,000lb to overcome the "breakaway force" and engrave the bullet but only 10,000lb THEREAFTER to keep it sliding. This all has nothing to do with whether or not the bullet is in or out of the lands.
Here are some of the reasons that the whole contention is flawed, and WHY I simply couldn't understand what Nick was saying, nor WHY he was saying them.
----Sliding the bullet out of the case neck certainly doesn't take 10,000lb! Most often the primer takes care of that little detail.
----Sliding the bullet down the freebore until it touches the leade certainly doesn't take 10,000lb!!!
----"Sliding" ...... the whole term "sliding" is the root of the problem here. Nick's illustration is based on the contention that once you've started that bullet moving it just keeps on "sliding" right into the lands and out the bore. THINK about this guys!
----So you DO slide the bullet out of the neck and "down the freebore and into the lands", maybe the bullet has even picked up a few FPS velocity.........So you give that bullet a whole 30thousandths of running start.........do you think it just KEEPS GOING into the lands without slowing down??? Do you REALLY think that that bullet has achieved so much velocity and inertia that it just SLITHERS into the lands like a greased eel???
NOT!
It picks up a LITTLE inertia, a LITTLE easement.......like to the tune of 1000psi/30thousandths....that's only 333lb for a difference of ten thou! So Let 'Er BUCK!!! Back off 60thou.......and maybe you'll get 2000lb or so of easement......WHOOPEEEEEEEE!!!
----You most certainly cannot jam a bullet deeply enough with the palm of your hand to completely eliminate the start-stop-start cycle, you can only tweak with it, tweaking the initial pressure curve incrementally. So even a bullet seated "deeply into the lands" is gonna' start-stop-start......
In the real world all the freebore does is slightly EASE the transition by giving the bullet a LITTLE inertia........You can EASE the transition into the lands just a wee bit, maybe UP TO a grand total of 5%.............1000psi or maybe even 1500psi TOTAL DIFFERENCE........ THE BULLET STILL STOPS unless you've given it a tremendous freerun boost of energy.
In fact, in the real world of accuracy shooting where the bullet is typically given 10thou or less of free-run, the bullet STARTS by slipping forward in the case neck.... then it STOPS in the leade.... and then STARTS AGAIN once pressure builds enough to actually engrave it. ALL within that .010-.020 distance between seated depth and actual engraving jam...........So what you REALLY get from all this 5 to 10 thou out stuff is only a teensy-weensy start-stop-start glitch effect. This does act to slightly slow down or actually stretch out the initial burn curve. The bullet gets jammed pretty deeply before it stops and the act of jamming it increases the size of the pressure vessel, not a huge increase but certainly an ENORMOUS increase in volume compared to "stretching effects of the chamber and the bore" and such........ Incidentally, this start-stop-start effect and its effect on the apparent size of the pressure chamber (case) is the BASIS for the whole concept of tuning with seating depth......tuning depth affects the initial burn cycle. It doesn't "spike" it nor make anything dangerous it just changes it, 'tweaks" it. And when you've got accurate equipment (NOT XC accurate, NOT Mid Thompkins accurate but BENCHREST accurate) then it can make a difference in the tune. A difference of ten or twenty thousandths on the target, 100yds away. Changing seating depth on a factory rig is an exercise in futility unless you've masochistic tendencies or lots of time to waste........For factory rifles there's IN the lands and OUT of the lands......I shoot all of my factory rifles off of a firm jam. That's it, no need for anything else. Just enough jam to help center into the bore yet loose enough not to pull a bullet.
Soooooooo, I at least THINK I've located the spot where Nick's interpretation of Vaughn's data led him to an erroneous assumption which of course leads inexorably to erroneous conclusion.
OR NOT!!!
Maybe it leads to Al's WRONG and Nick's RIGHT!!! That once that little ol' bullet slips in the neck it's well and truly SLIDING its way to a seamless exit....
LOL
al
I'm not sure how to "answer" all of this as it's getting a little muddled
But that's never kept me from trying 
I'll start with Nicks interpretation of my answers........
1.) Harold Vaughn measured a bullet engraving pressure at 1000 to 1500 psi, and that is not enough force to account for the 22% peak pressure increase on the graphs.
2.) No velocity increase appeared, so no pressure increase could have occurred.
3.) A number of people have had difficulty using the Pressure Trace instrument, so a readout from it should not be trusted.
In fact NONE of these assertions are mine.... I assume this to be a case of misunderstanding and not purposeful obfuscation of he issues.
#1--- Harold Vaughn did NOT "measure a bullet engraving pressure at 1000 to 1500 psi", I noted this as the total conceivable DIFFERENCE between touching and 30thou out of the lands. Bullet engraving pressure is in the neighborhood of 10,000psi, this from Vaughn, Rinker and Howell. Furthermore confusing is the fact that Nick goes on to restate WITH DETAIL and corroboration from Vaughn EXACTLY what I was saying........??? ......I'm not sure what to make of it.......
#2-- "No velocity increase appeared, so no pressure increase could have occurred." Again, simply a misrepresentation. I make many references to pressure and velocity which may all be encapsulated by this phrase from my post, "Pressure/velocity are essentially linear". "Essentially linear" is exactly what I meant to say, with "essentially" acting as a modifier to "linear". Velocity = pressure x area x time x (gravitational constant/mass) I'm not sure why G is thrown into the mix as it seems normally to be acting at right angles to the involved forces.......you can eliminate gravity and still come up with a valid result. IMO ignoring G is LESS a problem than ignoring non-linear combustion. (again, I'm happy to be SHOWN to be wrong )
#3-- Regarding RSI data, I have no opinion regarding "people having difficulty" with RSI equipment. I have difficulty with the conclusions drawn, mostly conclusions stated as fact by the folks at RSI.
Now regarding all of the fluff about those loads which exhibit pressure increase WITHOUT a commensurate velocity increase........ Again we're getting all mixed up here. The references (mainly anecdotal) to the "plateau" which often occurs when the reloader ventures above max loading capacity are used to somehow "support?" the argument???...... the referenced, while existent, is irrelevant to this discussion. The plateau is simply the result of factors colluding to change the burn rate of the powder. Nick alludes to all of this in his explanation but doesn't apply it to THIS discussion which is quite simply stated as, "CAN letting a bullet inadvertently touch the lands cause a dangerous pressure event".......... My answer is "NO."
To agree with a lot of what Nick is saying, "can events collaborate to CHANGE THE BURN RATE of the powder charge and cause non-linear progression of Pressure/Velocity?" Absolutely YES..............but simply touching the lands ain't good for a 20-25% increase in chamber pressure. And IF true chamber pressure were to increase 20-25% then there WOULD be an accompanant velocity spike.
Some other random quotes from Nick which I disagree with.
---"The short answer is that under higher pressure the gun metal exhibits greater strain and the powder burns hotter and the bore is stretched and distorted more." . . . . . . .Yeahh, SO??? We're looking at a total difference, energy conversion, in the area of 1-2% and this difference is incremental and linear. The "bore stretch" one is REALLY a stretch! since peak pressure occurs in the first inches of the bore.....and pressure is dropping from there on...... The real answer is that the burning pressure curve is distorted to become slightly less efficient, yes it DOES waste energy in heat and excessive vibration but not from a volumetric change in the rifle. We have a TRANSFER disconnect, not a distortion problem. Distortion is a symptom. This is really no different than the energy transfer of a bat to a ball, a bow to an arrow or a throwing arm to a rock.
Statements like this one just bug me....... Nick quotes Vaughn in ref to engraving force changing with caliber saying "That is not perfectly true, as I'll try to show shortly, but not way off the beam, either." WHY restate Vaughn in a attempt to steal his thunder? The REASON that Vaughn doesn't go on to detail the differences is that it's NOT RELEVANT! It's certainly "true enough" or accurate enough to make the point!
Or this ............ "If you are running a gun near its individual maximum pressure, it is often already a good bit above the SAAMI standard maximum, so letting a bullet touch the lands to give you another 5, 10 or 15 thousand psi or so, depending on the powder, is still enough to move a hot load up into proof pressure range, and that can't be good for the gun to have a steady diet of those. It won't give best accuracy anyway." ............... WHO SEZ it "won't give best accuracy anyways?" ALL competitive Bench Rest shooters work in this range and it's the ONLY way to be competitive and furthermore these actions have taken many times more "abusive pressure levels" in their short lives than any other type of rifle will take over time. "Time" is relative. The implication here is that firing these hot loads "over time" will incrementally damage the action. Simply not true. (BTW, using Mid Tompkins as a reference shows nothing but misunderstanding of the question. Middleton has neither the desire nor the capacity to achieve truly high-level intrinsic accuracy. You might as well use GD Tubb...... or Carlos Hathcock.......or Chuck Mawhinney)
And 90gr bullets "common for PPC"? and loaded to 34,000psi???? AGAIN simply no frame of reference. This is BRC for crying out loud. NO other discipline has the same level of intrinsic accuracy nor do other shooters run at the pressures that PPC/BR and now the 6.5X47L case run. ROUTINELY running pressures that are often touted as "action-destroying". BTW, H322 and a 90gr bullet is almost certainly a recipe for disaster! 322 is 'wayy too fast for that hunormous bullet. Not really relevant, but lest anyone be misled by casual verbiage.........and the level of ignorance that led RSI to use such a mismatch???? odd
Seems like Nick views the firing event sequence essentially as I do, no quarrels there..
We surely do agree re the safety aspects of all this. Be careful out there 
I still maintain that there's a disconnect somewhere betwixt the data and the interp........work needs to be done before certain things are stated as "fact".
________________________________________________________________
Al posted again -
This post has been bugging me so I just had to come back and reread it all the way thru.........
Regarding my posts and a person from another forum named "Nick" there has been kindofa' long distance third person back-n-forth regarding the mechanics of bullet startup and the use of RSI Pressure Trace equipment.
Now this "Nick" seems a knowledgeable guy and I consider myself to be fairly well-versed on the subjects presented yet we seem to have a disagreement. Discussion GOOD, disagreement, CONFUSING......... I've been confused by several of Nick's contentions.
I MAY have found one of our disconnects.
Nick uses this quote >>>> "In other words, the difference in start pressures between touching the lands (static case) or not touching the lands (kinetic, or sliding case) would also be about 10,000 psi." <<<<< as a way of restating some of the things that Harold Vaughn talks about in his book "Rifle Accuracy Facts".
The quote is taken from this paragraph here >>>>> 1.) "The first point is the easiest because it is an error caused by something that plagues me increasingly as the years go by, misremembered facts. What Harold Vaughn actually measured was not the pressure needed to engrave a bullet, but the force. Vaughn (p.29, 2nd ed.), describes pushing a .270 bullet into rifling with a hydraulic press equipped with a gage that measured 1200 pounds force was needed. To find the pressure needed to supply that force, you must divide the force by the cross-sectional area of the bullet. A .270 bullet has a cross-sectional area of 0.0603 in². Dividing that into 1200 lbs equals about 20,000 lbs/in² (psi). Since he was measuring the static case, Vaughn estimated half that would be required in the kinetic case, or 600 pounds force, which, in Vaughn's own words “. . . translates to a pressure on the base of the bullet of about 10,000 psi”. In other words, the difference in start pressures between touching the lands (static case) or not touching the lands (kinetic, or sliding case) would also be about 10,000 psi." <<<<<
HERE I think is where Nick and I really part ways, HERE is where I believe Nick MIS-INTERPRETS what Harold is saying and is led down a path of confusion.
Nick takes this to mean that the difference between touching and not touching is 10,000psi.
WRONG! This isn't what Harold's saying A'tall.......
THIS INTERPRETATION is flawed.............. the entire CONCEPT is flawed .......... I missed this in the first go-'round because I just can't always find another person's frame of reference. The problem is mine, I should have read it over until I realized what Nick was trying to say.
What Harold Vaughn is saying is that it takes 20,000lb to overcome the "breakaway force" and engrave the bullet but only 10,000lb THEREAFTER to keep it sliding. This all has nothing to do with whether or not the bullet is in or out of the lands.
Here are some of the reasons that the whole contention is flawed, and WHY I simply couldn't understand what Nick was saying, nor WHY he was saying them.
----Sliding the bullet out of the case neck certainly doesn't take 10,000lb! Most often the primer takes care of that little detail.
----Sliding the bullet down the freebore until it touches the leade certainly doesn't take 10,000lb!!!
----"Sliding" ...... the whole term "sliding" is the root of the problem here. Nick's illustration is based on the contention that once you've started that bullet moving it just keeps on "sliding" right into the lands and out the bore. THINK about this guys!
----So you DO slide the bullet out of the neck and "down the freebore and into the lands", maybe the bullet has even picked up a few FPS velocity.........So you give that bullet a whole 30thousandths of running start.........do you think it just KEEPS GOING into the lands without slowing down??? Do you REALLY think that that bullet has achieved so much velocity and inertia that it just SLITHERS into the lands like a greased eel???
NOT!
It picks up a LITTLE inertia, a LITTLE easement.......like to the tune of 1000psi/30thousandths....that's only 333lb for a difference of ten thou! So Let 'Er BUCK!!! Back off 60thou.......and maybe you'll get 2000lb or so of easement......WHOOPEEEEEEEE!!!
----You most certainly cannot jam a bullet deeply enough with the palm of your hand to completely eliminate the start-stop-start cycle, you can only tweak with it, tweaking the initial pressure curve incrementally. So even a bullet seated "deeply into the lands" is gonna' start-stop-start......
In the real world all the freebore does is slightly EASE the transition by giving the bullet a LITTLE inertia........You can EASE the transition into the lands just a wee bit, maybe UP TO a grand total of 5%.............1000psi or maybe even 1500psi TOTAL DIFFERENCE........ THE BULLET STILL STOPS unless you've given it a tremendous freerun boost of energy.
In fact, in the real world of accuracy shooting where the bullet is typically given 10thou or less of free-run, the bullet STARTS by slipping forward in the case neck.... then it STOPS in the leade.... and then STARTS AGAIN once pressure builds enough to actually engrave it. ALL within that .010-.020 distance between seated depth and actual engraving jam...........So what you REALLY get from all this 5 to 10 thou out stuff is only a teensy-weensy start-stop-start glitch effect. This does act to slightly slow down or actually stretch out the initial burn curve. The bullet gets jammed pretty deeply before it stops and the act of jamming it increases the size of the pressure vessel, not a huge increase but certainly an ENORMOUS increase in volume compared to "stretching effects of the chamber and the bore" and such........ Incidentally, this start-stop-start effect and its effect on the apparent size of the pressure chamber (case) is the BASIS for the whole concept of tuning with seating depth......tuning depth affects the initial burn cycle. It doesn't "spike" it nor make anything dangerous it just changes it, 'tweaks" it. And when you've got accurate equipment (NOT XC accurate, NOT Mid Thompkins accurate but BENCHREST accurate) then it can make a difference in the tune. A difference of ten or twenty thousandths on the target, 100yds away. Changing seating depth on a factory rig is an exercise in futility unless you've masochistic tendencies or lots of time to waste........For factory rifles there's IN the lands and OUT of the lands......I shoot all of my factory rifles off of a firm jam. That's it, no need for anything else. Just enough jam to help center into the bore yet loose enough not to pull a bullet.
Soooooooo, I at least THINK I've located the spot where Nick's interpretation of Vaughn's data led him to an erroneous assumption which of course leads inexorably to erroneous conclusion.
OR NOT!!!
Maybe it leads to Al's WRONG and Nick's RIGHT!!! That once that little ol' bullet slips in the neck it's well and truly SLIDING its way to a seamless exit....
LOL
al
Bullet94,
Something to send to Al:
Al wrote: "Velocity = pressure x area x time x (gravitational constant/mass)I'm not sure why G is thrown into the mix as it seems normally to be acting at right angles to the involved forces.......you can eliminate gravity and still come up with a valid result. IMO ignoring G is LESS a problem than ignoring non-linear combustion. (again, I'm happy to be SHOWN to be wrong )"
The reason the "G" is used in equations that relate mass and velocity to force applied is a little complicted for people who use the English system to understand, because "pounds" is used in the English system in two different ways. (People used to the metric system don't have this confusion, because different words are used for force and mass.)
In both systems, force = mass x acceleration. In the English system, the proper units are force in pounds, acceleration in ft/sec^2 and the mass in SLUGS. One pound of force will accelerate one slug at a rate of one foot per second each second that force is applied. But, non-technical people talk about pounds of stuff (like lead) as if it were the mass of the lead, but it is really the vertical force of the lead acting on a scale in the presence of earth's gravitational force, which produces an acceleration force on everyting (on earth's surface) of 32.2 ft/sec^2. So, a slug of material will weigh 32.2 pounds when sitting on the earth's surface, because it is experiencing the force of graivty that would accelerate it at 32.2 ft/sec^2. So, a lump of lead that weighs one pound is 1/32.2 of a slug of lead. A horizontal force of one pound on 1/32.2 slug of material will accelerate it at 32.2 ft/sec^2 in the horizontal direction, not 1 ft/sec^2. So, if we want to know how much acceleration we will get by putting a one pound force from gas pressure on the base of bullet that weighs one pound on earth's surface, we need to convert the mass of the bullet to the proper mass unit for the English system, which is slugs. Dividing the mass of the bullet by G (32.2, roughly) to get slugs then produces the correct acceleration.
SL1
Something to send to Al:
Al wrote: "Velocity = pressure x area x time x (gravitational constant/mass)I'm not sure why G is thrown into the mix as it seems normally to be acting at right angles to the involved forces.......you can eliminate gravity and still come up with a valid result. IMO ignoring G is LESS a problem than ignoring non-linear combustion. (again, I'm happy to be SHOWN to be wrong )"
The reason the "G" is used in equations that relate mass and velocity to force applied is a little complicted for people who use the English system to understand, because "pounds" is used in the English system in two different ways. (People used to the metric system don't have this confusion, because different words are used for force and mass.)
In both systems, force = mass x acceleration. In the English system, the proper units are force in pounds, acceleration in ft/sec^2 and the mass in SLUGS. One pound of force will accelerate one slug at a rate of one foot per second each second that force is applied. But, non-technical people talk about pounds of stuff (like lead) as if it were the mass of the lead, but it is really the vertical force of the lead acting on a scale in the presence of earth's gravitational force, which produces an acceleration force on everyting (on earth's surface) of 32.2 ft/sec^2. So, a slug of material will weigh 32.2 pounds when sitting on the earth's surface, because it is experiencing the force of graivty that would accelerate it at 32.2 ft/sec^2. So, a lump of lead that weighs one pound is 1/32.2 of a slug of lead. A horizontal force of one pound on 1/32.2 slug of material will accelerate it at 32.2 ft/sec^2 in the horizontal direction, not 1 ft/sec^2. So, if we want to know how much acceleration we will get by putting a one pound force from gas pressure on the base of bullet that weighs one pound on earth's surface, we need to convert the mass of the bullet to the proper mass unit for the English system, which is slugs. Dividing the mass of the bullet by G (32.2, roughly) to get slugs then produces the correct acceleration.
SL1
Response part one
Bullet94,
I'll break this response into two or three parts. This first will just deal with the initial issue which was whether or not Al was right that a difference of only 1000 psi would result from a bullet touching the lands. I'll address the other points separately, as time allows, for anyone who is still interested. This is in order to keep the post character count within its limits.
First we need to clarify something with Al:
Not at this end. Al is posting in a bench rest shooting forum but I am not, except indirectly through Bullet94's copying of the posts between here and there. Ours is a general firearms forum. So when Al gets exercised because I mention Mid Tompkins, whose shooting he sees as irrelevant because it is not BR, that is so only at his end. Tompkins's loading technique is an example of properly developed loads that touch the lands and are useful for their purpose. I merely used him to be sure people reading the post understood that touching the lands intentionally is not always bad. I suspect Al would have preferred I had used Harold Vaughn's reference to this technique? On page 36-37 of Rifle Accuracy Facts, 2nd Edition, Vaughn says:
I should add, since Vaughn puts faith in it, that “Reference 1”, above, is a book called Absolute Chamber Pressure in Center Fire Rifles, by Brownell, York. Sinderman, Jacobs, and Robbins, University of Michigan, Ann Arbor, Michigan. 1965. That is awhile ago, but it gives you the source of the numbers Vaughn believes. If you can find a copy, you can look them up.
Well, let me quote from Al's first reply to appear on this forum. This is what I took assertion 1.) from:
This is what I took assertions 2.) and 3.) from:
This is where Al reinforced assertion 3.):
If I somehow misunderstood those statements, I'll leave it to Al to explain them and how they do not comprise his assertions?
No. It was not “EXACTLY” what Al was saying. That was the whole point of my posting it. Al misused Vaughn's units. I went through the calculations using the units correctly to show the correct result is almost 17 times more pressure than Al claimed Vaughn's measurement represents. Al said the difference in pressure created by a bullet touching the lands was “1000 psi”. 1000 psi produces only 60 lbs of force on a .277” diameter bullet; enough to seat a bullet in a case, but not enough to engrave squat.
Yes, it is exactly what Vaughn is saying if you know what the units mean. That is what brings us to the basis of this whole disagreement. In my first post I tried to leave Al some wiggle room when I suggested he misremembered the data and didn't point out his unit inconsistencies. Al does not seem to have been able to follow that, so I will resort to being blunt. The complete and accurate quote from Al's first post, with bold type added by me to emphasize the inconsistency was:
From that statement, it is apparent that Al believes psi and lbs mean the same thing and are interchangeable. They do not, and are not. Pounds are units of force, and psi are units of pressure. Pressure is the distribution of force, that is, how much force is applied a whole unit of area, not how much force is applied to the small portion of that area a bullet or bore represents. In this case, pressure is the number of pounds force seen by each full square inch of chamber, including the small portion the bullet base occupies. Most bullets are nowhere near a whole square inch in cross-sectional area, so the number of those pounds of force it intercepts is a fraction of what a whole square inch sees, and is therefore a fraction of the magnitude (size) of the pressure number. 1,000 psi, Al's first number, would apply only about 60 lbs of force to a .270 bullet, while 1500 lbs force, Al's second number, would require about 25,000 psi to apply it to that same bullet. Force and pressure numbers would only be the same if the bullet were a full square inch in cross-sectional area (about 113 caliber).
Mechanics who work with hydraulics and pneumatics often drop “per square inch” to shorten “pounds per square inch” just to “pounds” in their vernacular. But such slang does not allow accurate calculations to be made where more than one unit is involved. That is why neither physicists nor Harold Vaughn truncate the terminology in that way. Al clearly does not understand the basic physics units and their the pressure and force relationship. He quite openly states he doesn't understand why the gravitational constant plays a roll in the relationship of units of weight and mass and force. Understanding units and familiarity with how units work in calculations are tools necessary to calculate pressure accurately. Without them you can only get correct results by accident.
I don't intend to write a basic physics text here. If anyone else reading this missed the chance to take basic physics in high school, and wants to learn what's needed for calculations of the type described here, I would recommend getting a copy of The Cartoon Guide to Physics (no, that's not a joke, it is the actual title, and the book does have lots of non-threatening cartoon illustrations) by Larry Gonick. It is a relatively easily understood introductory physics book.
Not. First, again, as in Al's other posts, the units are wrong. 20,000lb, would require 331,800 psi acting on Vaughn's bullet. 10,000 lbs would need 165,900 psi. But assuming Al meant to say 20,000 psi and 10,000 psi, then what's his point? Does he think a drop in friction will drop pressure? Once the compressed gas is present to make 20,000 psi, it would require Captain Kirk beaming half the gas out of the chamber at the moment the bullet started moving to drop that pressure in half. I am not a member of Star Fleet, but I am reasonably confident that isn't being done. Once the bullet starts to move, it is not merely kept sliding, but is also rapidly accelerated. Under rapid acceleration, the main resistance against which burning powder builds pressure is the reaction force presented by the inertia of the bullet mass, not that of the friction. When gas pressure builds to 20,000 psi to start a bullet moving, the burning powder just keeps building pressure from there until the peak is reached.
The role of start pressure in this discussion has been that it is different when the bullet touches the lands at the throat than when it does not. In the former case the value is higher. Subsequent sliding friction isn't relevant to that. Higher start pressure causes the chamber pressure peak to be reached earlier in the bullet's travel time down the bore. This is because the added time needed to build to that higher start pressure has allowed more powder to get burning before the bullet began to move. That's the additional hesitation I referred to in my earlier post that is above and beyond the hesitation in the neck of a bullet seated off the lands. The higher start pressure will accelerate the sliding bullet faster in the early part of its travel, so the bullet distance down the bore at peak pressure will not shorten by much. It will, however, shorten a little and contribute an additional couple of percent or so to the pressure peak above and beyond the difference in start pressures. It is important to keep in mind the difference in start pressure is not the end concern here, but rather it is the difference in peak pressure that results from the difference in start pressure.
To summarize, in this post I have shown Al's pressure difference numbers were wrong because he doesn't understand how the units work, resulting in his assumption that force numbers were the same magnitude as pressure numbers. In reality these are almost 17 times bigger in the case of the .277" bullets. They are almost 22 times bigger for 6 mm. Once that is understood, the 22% gain in pressure shown in the plots I included in my first post fall well within that range and are perfectly reasonable. In the prior post I showed the pressure plots were accurate. Either clipping the bottom or introducing a non-linear exaggeration of peak values would require the muzzle pressure values shown to be unrealistically low. Instead the pressure shown at the end of the plots are corroborated by computer modeling.
Nick
Bullet94,
I'll break this response into two or three parts. This first will just deal with the initial issue which was whether or not Al was right that a difference of only 1000 psi would result from a bullet touching the lands. I'll address the other points separately, as time allows, for anyone who is still interested. This is in order to keep the post character count within its limits.
First we need to clarify something with Al:
Al said:“This is BRC for crying out loud.”
Not at this end. Al is posting in a bench rest shooting forum but I am not, except indirectly through Bullet94's copying of the posts between here and there. Ours is a general firearms forum. So when Al gets exercised because I mention Mid Tompkins, whose shooting he sees as irrelevant because it is not BR, that is so only at his end. Tompkins's loading technique is an example of properly developed loads that touch the lands and are useful for their purpose. I merely used him to be sure people reading the post understood that touching the lands intentionally is not always bad. I suspect Al would have preferred I had used Harold Vaughn's reference to this technique? On page 36-37 of Rifle Accuracy Facts, 2nd Edition, Vaughn says:
”Seating depth of a bullet in a case has an effect on just how close to the center a bullet will line up. Obviously, the bullet will be centered if it is in complete contact with the lands, however Reference 1 showed that peak chamber pressure decreases if a bullet has a free run before it contacts the lands. Since a minimum in peak pressure for a given load implies minimum bullet distortion, the author prefers a seating depth that will provide about 0.010 inches into the lands in the case of a bench rest gun with light bullets and about 0.020” off the lands in the case of a sporter using heavier bullets.”
I should add, since Vaughn puts faith in it, that “Reference 1”, above, is a book called Absolute Chamber Pressure in Center Fire Rifles, by Brownell, York. Sinderman, Jacobs, and Robbins, University of Michigan, Ann Arbor, Michigan. 1965. That is awhile ago, but it gives you the source of the numbers Vaughn believes. If you can find a copy, you can look them up.
Al said:I'll start with Nicks interpretation of my answers........
1.) Harold Vaughn measured a bullet engraving pressure at 1000 to 1500 psi, and that is not enough force to account for the 22% peak pressure increase on the graphs.
2.) No velocity increase appeared, so no pressure increase could have occurred.
3.) A number of people have had difficulty using the Pressure Trace instrument, so a readout from it should not be trusted.
In fact NONE of these assertions are mine....
Well, let me quote from Al's first reply to appear on this forum. This is what I took assertion 1.) from:
Al said:Without going into endless detail, suffice it to say that theoretical data agree with directly MEASURED data in this instance. The real difference between thirty thou out and touching the lands is around 1000psi.
This is what I took assertions 2.) and 3.) from:
Al said:I've had calls and letters from quite a few folks who own RSI systems. They're constantly confronted with "pressure trace" readings which show HUGE increases in pressure while reading only modest increases in velocity. A difference of 25% from one shot to the next isn't "modest"!! A 25% increase in pressure is hellacious..........and should result in a velocity increase to match the pressure spike.
This is where Al reinforced assertion 3.):
Al said:.......far from "dangerous" and FAR from "25%". It seems that the data presented by the RSI pressure traces increases this effect by a factor of 10!
If I somehow misunderstood those statements, I'll leave it to Al to explain them and how they do not comprise his assertions?
Al said:Furthermore confusing is the fact that Nick goes on to restate WITH DETAIL and corroboration from Vaughn EXACTLY what I was saying........??? ......I'm not sure what to make of it.......
No. It was not “EXACTLY” what Al was saying. That was the whole point of my posting it. Al misused Vaughn's units. I went through the calculations using the units correctly to show the correct result is almost 17 times more pressure than Al claimed Vaughn's measurement represents. Al said the difference in pressure created by a bullet touching the lands was “1000 psi”. 1000 psi produces only 60 lbs of force on a .277” diameter bullet; enough to seat a bullet in a case, but not enough to engrave squat.
Al said:Nick takes this to mean that the difference between touching and not touching is 10,000psi.
WRONG! This isn't what Harold's saying A'tall.......
Yes, it is exactly what Vaughn is saying if you know what the units mean. That is what brings us to the basis of this whole disagreement. In my first post I tried to leave Al some wiggle room when I suggested he misremembered the data and didn't point out his unit inconsistencies. Al does not seem to have been able to follow that, so I will resort to being blunt. The complete and accurate quote from Al's first post, with bold type added by me to emphasize the inconsistency was:
Al said:Without going into endless detail, suffice it to say that theoretical data agree with directly MEASURED data in this instance. The real difference between thirty thou out and touching the lands is around 1000psi. The maximum difference that one could realize would not exceed 1500lb.
From that statement, it is apparent that Al believes psi and lbs mean the same thing and are interchangeable. They do not, and are not. Pounds are units of force, and psi are units of pressure. Pressure is the distribution of force, that is, how much force is applied a whole unit of area, not how much force is applied to the small portion of that area a bullet or bore represents. In this case, pressure is the number of pounds force seen by each full square inch of chamber, including the small portion the bullet base occupies. Most bullets are nowhere near a whole square inch in cross-sectional area, so the number of those pounds of force it intercepts is a fraction of what a whole square inch sees, and is therefore a fraction of the magnitude (size) of the pressure number. 1,000 psi, Al's first number, would apply only about 60 lbs of force to a .270 bullet, while 1500 lbs force, Al's second number, would require about 25,000 psi to apply it to that same bullet. Force and pressure numbers would only be the same if the bullet were a full square inch in cross-sectional area (about 113 caliber).
Mechanics who work with hydraulics and pneumatics often drop “per square inch” to shorten “pounds per square inch” just to “pounds” in their vernacular. But such slang does not allow accurate calculations to be made where more than one unit is involved. That is why neither physicists nor Harold Vaughn truncate the terminology in that way. Al clearly does not understand the basic physics units and their the pressure and force relationship. He quite openly states he doesn't understand why the gravitational constant plays a roll in the relationship of units of weight and mass and force. Understanding units and familiarity with how units work in calculations are tools necessary to calculate pressure accurately. Without them you can only get correct results by accident.
I don't intend to write a basic physics text here. If anyone else reading this missed the chance to take basic physics in high school, and wants to learn what's needed for calculations of the type described here, I would recommend getting a copy of The Cartoon Guide to Physics (no, that's not a joke, it is the actual title, and the book does have lots of non-threatening cartoon illustrations) by Larry Gonick. It is a relatively easily understood introductory physics book.
Al said:What Harold Vaughn is saying is that it takes 20,000lb to overcome the "breakaway force" and engrave the bullet but only 10,000lb THEREAFTER to keep it sliding. This all has nothing to do with whether or not the bullet is in or out of the lands. . .
. . . Soooooooo, I at least THINK I've located the spot where Nick's interpretation of Vaughn's data led him to an erroneous assumption which of course leads inexorably to erroneous conclusion.
OR NOT!!!
Not. First, again, as in Al's other posts, the units are wrong. 20,000lb, would require 331,800 psi acting on Vaughn's bullet. 10,000 lbs would need 165,900 psi. But assuming Al meant to say 20,000 psi and 10,000 psi, then what's his point? Does he think a drop in friction will drop pressure? Once the compressed gas is present to make 20,000 psi, it would require Captain Kirk beaming half the gas out of the chamber at the moment the bullet started moving to drop that pressure in half. I am not a member of Star Fleet, but I am reasonably confident that isn't being done. Once the bullet starts to move, it is not merely kept sliding, but is also rapidly accelerated. Under rapid acceleration, the main resistance against which burning powder builds pressure is the reaction force presented by the inertia of the bullet mass, not that of the friction. When gas pressure builds to 20,000 psi to start a bullet moving, the burning powder just keeps building pressure from there until the peak is reached.
The role of start pressure in this discussion has been that it is different when the bullet touches the lands at the throat than when it does not. In the former case the value is higher. Subsequent sliding friction isn't relevant to that. Higher start pressure causes the chamber pressure peak to be reached earlier in the bullet's travel time down the bore. This is because the added time needed to build to that higher start pressure has allowed more powder to get burning before the bullet began to move. That's the additional hesitation I referred to in my earlier post that is above and beyond the hesitation in the neck of a bullet seated off the lands. The higher start pressure will accelerate the sliding bullet faster in the early part of its travel, so the bullet distance down the bore at peak pressure will not shorten by much. It will, however, shorten a little and contribute an additional couple of percent or so to the pressure peak above and beyond the difference in start pressures. It is important to keep in mind the difference in start pressure is not the end concern here, but rather it is the difference in peak pressure that results from the difference in start pressure.
To summarize, in this post I have shown Al's pressure difference numbers were wrong because he doesn't understand how the units work, resulting in his assumption that force numbers were the same magnitude as pressure numbers. In reality these are almost 17 times bigger in the case of the .277" bullets. They are almost 22 times bigger for 6 mm. Once that is understood, the 22% gain in pressure shown in the plots I included in my first post fall well within that range and are perfectly reasonable. In the prior post I showed the pressure plots were accurate. Either clipping the bottom or introducing a non-linear exaggeration of peak values would require the muzzle pressure values shown to be unrealistically low. Instead the pressure shown at the end of the plots are corroborated by computer modeling.
Nick
Unclenick
I'll break this response into two or three parts. This first will just deal with the initial issue which was whether or not Al was right that a difference of only 1000 psi would result from a bullet touching the lands. I'll address the other points separately, as time allows, for anyone who is still interested. This is in order to keep the post character count within its limits.
Unclenick
I’m still interested. I think this is one of the best threads I have read.
Al's reply -
Nice try
but NOPE!
I was not misunderstanding psi and pounds........I just used lb in the same sentence to mean the same thing as psi. "lb" is understood as psi. I fully understand the application. Just substitute "psi" where I short-handed it as "lb" and it all comes together. I used "pounds" to mean "pounds/square inch" because I AM a mechanic............
Here--- "The real difference between thirty thou out and touching the lands is around 1000psi. The maximum difference that one could realize would not exceed 1500lb."
lb should read psi. I said it ON PURPOSE, not because I "misunderstood it" as Nick states....
"From that statement, it is apparent that Al believes psi and lbs mean the same thing and are interchangeable. They do not, and are not. Pounds are units of force, and psi are units of pressure."
Nick is right.........psi and lb are NOT the same thing but this has no relevance to the original question
I'm a slob, I used "lb" inadvisably where i should have just stuck with "psi" but I did it ONLY for grammatical reasons, i used it ONLY in the interest of making for a more interesting read. In the same sentence I may well use "thousandths" and "thou" and "mil" interchangeably also but only because it's boring to use the same descriptive term over and over.
SOOOO, if I were to take the time to go back and re-write the post putting in the correct "psi" where I used "lb" as shorthand then Nick would read it differently BUT nothing would change.
The FACT is that seating depth variations up to 40 thou will only change the PSI generated 1000-1500 PSI which as Nick pointed out equals only 60-90 LB of force................negligible. (and 40thou is a TON of change, most guys play around in the "touching to .010-out range".
I DID and DO understand the units. Nicks post inadvertantly proves me right. When Nick runs the numbers (which he shows himself capable of doing) he SEES that the difference is under 100 lb of actual force exerted to the rear of the bullet. 1000-2000psi is DIDDLEY-SQUAT and this quite simply IS the number we're working with. BTW all of this "factor if 17" stuff and "331,800 psi" stuff is just goofy-speak. I short-handed the terms Nick, that's ALL....
((((((( BTW, Nick KNOWS THIS as proven by this statement "Mechanics who work with hydraulics and pneumatics often drop “per square inch” to shorten “pounds per square inch” just to “pounds” in their vernacular. But such slang does not allow accurate calculations to be made where more than one unit is involved. )))))))
I perticklerly like Nick's statement here " Force and pressure numbers would only be the same if the bullet were a full square inch in cross-sectional area (about 113 caliber). " Well DUHHH Nick
In the interest of simplicity for those who DON'T know the math but have access to the tome being referenced, "Rifle Accuracy Facts" by Harold Vaughn, you can go check for yourself. There is only one printed version of this book available so I can ref by page number. Harold has a handy tendency to wrap up his chapters with a summary for those NOT inclined to do the math and here's what Harold says in his own words:
Page 29 titled "engraving force",
"The pressure change caused by changing bullet seating depth in the case can also be deduced from Reference 1 for 0.308 caliber bullets. The peak chamber pressure will drop about 1000 psi for every 30 mils of additional distance (free run) between the bullet and contact with the lands in the throat. In other words, if you seat the bullet so that it has about 60 mils of free run before contacting the lands, the peak chamber pressure will be reduced by about 2000psi. This means that the chamber pressure is not very sensitive to seating depth."
The term "mils" is mechspeak for thousandths. 60 mils is 60 thousandths or .060 which is a BUNCH of "free run".
And Nick WILL confirm that if one "does the math" he finds that even when jumping 60 thousandths VS touching the lands the difference in pressure is down in the dirt, lost in the noise........
I am GUILTY AS CHARGED for being cavalier with my terms but I'm STILL RIGHT
Now, this is where I get a liddle confused again....... By NICK, not by the math.
??????........ this statement by Nick seems to indicate that he DOES NOT understand his own math!
"Nick takes this to mean that the difference between touching and not touching is 10,000psi.
WRONG! This isn't what Harold's saying A'tall.......
Yes, it is exactly what Vaughn is saying if you know what the units mean. That is what brings us to the basis of this whole disagreement. In my first post I tried to leave Al some wiggle room when I suggested he misremembered the data and didn't point out his unit inconsistencies. Al does not seem to have been able to follow that, so I will resort to being blunt. The complete and accurate quote from Al's first post, with bold type added by me to emphasize the inconsistency was"
In fact, let's break it down even further.........
------------------------------------------------------------------------
"Nick takes this to mean that the difference between touching and not touching is 10,000psi.
WRONG! This isn't what Harold's saying A'tall.......
Yes, it is exactly what Vaughn is saying if you know what the units mean."
----------------------------------------------------------------------------
This SEEMS to read that Nick believes that the difference between touching and NOT touching IS IN FACT TEN THOUSAND PSI!!!
Nick, check your math. The difference between 30 thou out VS touching the lands is only 1000psi, not ten thousand.
OR..........50-70 POUNDS if you prefer
al
Nice try
but NOPE!
I was not misunderstanding psi and pounds........I just used lb in the same sentence to mean the same thing as psi. "lb" is understood as psi. I fully understand the application. Just substitute "psi" where I short-handed it as "lb" and it all comes together. I used "pounds" to mean "pounds/square inch" because I AM a mechanic............
Here--- "The real difference between thirty thou out and touching the lands is around 1000psi. The maximum difference that one could realize would not exceed 1500lb."
lb should read psi. I said it ON PURPOSE, not because I "misunderstood it" as Nick states....
"From that statement, it is apparent that Al believes psi and lbs mean the same thing and are interchangeable. They do not, and are not. Pounds are units of force, and psi are units of pressure."
Nick is right.........psi and lb are NOT the same thing but this has no relevance to the original question
I'm a slob, I used "lb" inadvisably where i should have just stuck with "psi" but I did it ONLY for grammatical reasons, i used it ONLY in the interest of making for a more interesting read. In the same sentence I may well use "thousandths" and "thou" and "mil" interchangeably also but only because it's boring to use the same descriptive term over and over.
SOOOO, if I were to take the time to go back and re-write the post putting in the correct "psi" where I used "lb" as shorthand then Nick would read it differently BUT nothing would change.
The FACT is that seating depth variations up to 40 thou will only change the PSI generated 1000-1500 PSI which as Nick pointed out equals only 60-90 LB of force................negligible. (and 40thou is a TON of change, most guys play around in the "touching to .010-out range".
I DID and DO understand the units. Nicks post inadvertantly proves me right. When Nick runs the numbers (which he shows himself capable of doing) he SEES that the difference is under 100 lb of actual force exerted to the rear of the bullet. 1000-2000psi is DIDDLEY-SQUAT and this quite simply IS the number we're working with. BTW all of this "factor if 17" stuff and "331,800 psi" stuff is just goofy-speak. I short-handed the terms Nick, that's ALL....
((((((( BTW, Nick KNOWS THIS as proven by this statement "Mechanics who work with hydraulics and pneumatics often drop “per square inch” to shorten “pounds per square inch” just to “pounds” in their vernacular. But such slang does not allow accurate calculations to be made where more than one unit is involved. )))))))
I perticklerly like Nick's statement here " Force and pressure numbers would only be the same if the bullet were a full square inch in cross-sectional area (about 113 caliber). " Well DUHHH Nick
In the interest of simplicity for those who DON'T know the math but have access to the tome being referenced, "Rifle Accuracy Facts" by Harold Vaughn, you can go check for yourself. There is only one printed version of this book available so I can ref by page number. Harold has a handy tendency to wrap up his chapters with a summary for those NOT inclined to do the math and here's what Harold says in his own words:
Page 29 titled "engraving force",
"The pressure change caused by changing bullet seating depth in the case can also be deduced from Reference 1 for 0.308 caliber bullets. The peak chamber pressure will drop about 1000 psi for every 30 mils of additional distance (free run) between the bullet and contact with the lands in the throat. In other words, if you seat the bullet so that it has about 60 mils of free run before contacting the lands, the peak chamber pressure will be reduced by about 2000psi. This means that the chamber pressure is not very sensitive to seating depth."
The term "mils" is mechspeak for thousandths. 60 mils is 60 thousandths or .060 which is a BUNCH of "free run".
And Nick WILL confirm that if one "does the math" he finds that even when jumping 60 thousandths VS touching the lands the difference in pressure is down in the dirt, lost in the noise........
I am GUILTY AS CHARGED for being cavalier with my terms but I'm STILL RIGHT
Now, this is where I get a liddle confused again....... By NICK, not by the math.
??????........ this statement by Nick seems to indicate that he DOES NOT understand his own math!
"Nick takes this to mean that the difference between touching and not touching is 10,000psi.
WRONG! This isn't what Harold's saying A'tall.......
Yes, it is exactly what Vaughn is saying if you know what the units mean. That is what brings us to the basis of this whole disagreement. In my first post I tried to leave Al some wiggle room when I suggested he misremembered the data and didn't point out his unit inconsistencies. Al does not seem to have been able to follow that, so I will resort to being blunt. The complete and accurate quote from Al's first post, with bold type added by me to emphasize the inconsistency was"
In fact, let's break it down even further.........
------------------------------------------------------------------------
"Nick takes this to mean that the difference between touching and not touching is 10,000psi.
WRONG! This isn't what Harold's saying A'tall.......
Yes, it is exactly what Vaughn is saying if you know what the units mean."
----------------------------------------------------------------------------
This SEEMS to read that Nick believes that the difference between touching and NOT touching IS IN FACT TEN THOUSAND PSI!!!
Nick, check your math. The difference between 30 thou out VS touching the lands is only 1000psi, not ten thousand.
OR..........50-70 POUNDS if you prefer
al
Did Vaughn state a force or a pressure?
It seems like PART of the answer here can be found by clearing up one point:
What is Vaughn's engraving force measurement, really? I don't have the reference book, so I have to ask. Unclenick wrote:
If that is indeed correct, then the PRESSURE needed to create that force on the bullet is as Unclenick calculates it. But, if Vaughn really measured a pressure, then Al could be correct. SO, what did Vaughn really measure?
And, why does Al say
while Unclenick is citing the second edition?
Let's clear-up this point before we go on to the effects of changing the start pressure on the peak pressure.
SL1
It seems like PART of the answer here can be found by clearing up one point:
What is Vaughn's engraving force measurement, really? I don't have the reference book, so I have to ask. Unclenick wrote:
Vaughn (p.29, 2nd ed.), describes pushing a .270 bullet into rifling with a hydraulic press equipped with a gage that measured 1200 pounds force was needed.
If that is indeed correct, then the PRESSURE needed to create that force on the bullet is as Unclenick calculates it. But, if Vaughn really measured a pressure, then Al could be correct. SO, what did Vaughn really measure?
And, why does Al say
There is only one printed version of this book available
while Unclenick is citing the second edition?
Let's clear-up this point before we go on to the effects of changing the start pressure on the peak pressure.
SL1
Since this was mentioned as used by Vaughn (Absolute Chamber Pressure in Center Fire Rifles, by Brownell, York. Sinderman, Jacobs, and Robbins, University of Michigan, Ann Arbor, Michigan. 1965) I went to their site
http://deepblue.lib.umich.edu/handle/2027.42/3866
http://deepblue.lib.umich.edu/bitstream/2027.42/3866/5/bac6873.0001.001.pdf
From what I read it looks like the testing done at the site above seems to me to say there is about a 10,000 psi difference between seating off the lands verses seating with contact with the lands, when the seating depth is increased (¼ inch longer). In another test it looks like gas leakage was responsible for 6,250psi difference without touching the lands. See page 48-51 in the PDF file from the link above.
http://deepblue.lib.umich.edu/handle/2027.42/3866
http://deepblue.lib.umich.edu/bitstream/2027.42/3866/5/bac6873.0001.001.pdf
From what I read it looks like the testing done at the site above seems to me to say there is about a 10,000 psi difference between seating off the lands verses seating with contact with the lands, when the seating depth is increased (¼ inch longer). In another test it looks like gas leakage was responsible for 6,250psi difference without touching the lands. See page 48-51 in the PDF file from the link above.
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SL1,
My copy says 2nd edition, printed in 2000. It is the only version you can buy new now, of course, but proper referencing requires the edition to be mentioned to avoid possible errors by persons looking in the 1998 edition. A search of Amazon.com turned up one copy of the 1998 edition, though I had first heard mention of it in comments by Dave Brennan, Precision Shooting's editor, referring to a possible follow-up book by Vaughn being in the works. I don't know what became of that project?
Vaughn measured 1200 lbs applied force in the static case. He then estimated that quantity would be halved in the kinetic case, to 600 lbs, which halved quantity, he stated, would take 10,000 psi to be produced on the .270 bullet. I believe, without looking back at it, that I quoted Vaughn directly on that point in my first response to Al's misreading of Vaughn's data. Doubling the force back to the static case would correspondingly double the required pressure to 20,000 lbs.
Al's explanation that he intentionally used units inconsistently, mixing psi and lb on purpose, is really not very satisfactory, but taking it at face value, then his error is in thinking a trained engineer and scientist like Vaughn would do the same thing. Vaughn did not. When Vaughn said he measured 1200 lbs in the static case, that's what he actually meant. Not 1200 psi. Otherwise he would not have subsequently referred to half that force, 600 lbs, requiring 10,000 psi acting on the bullet. 10,000 is, of course, a magnitude almost 17 times bigger than 600. So when Al says: "BTW all of this "factor if 17" stuff and "331,800 psi" stuff is just goofy-speak", it just shows, again, that he doesn't understand this relationship or the mathematical application of the units. I don't know how else to interpret that?
The other Vaughn statement Al referred to about pressure decline with additional seating depth (additional distance off the lands) applies only to one distance or another off the lands, and does not address the touching case. It is also true only over a limited range of distances off the lands. If a bullet is large enough and seated deeply enough, pressure starts to climb again due to reduction in combustion space at the start.
I am trying to get hold of a copy of "Reference 1", Absolute Chamber Pressure in Center Fire Rifles, the book whose numbers Vaughn himself believes on this topic. If I succeed, I'll report what it says?
P.S. Bullet 94,
Looks like I started composing and got interrupted and you posted the link in the meantime. Thanks for that. I thought of bringing up blow-by before, but it is not a terribly consistent behaving thing, varying with throat dimensions and style (ball vs. freebore) and the diameter of freebore where there is one, and, of course, bullet obturation, which is greater with the thin skinned match bullets than with solids, just to pick two extremes.
Glad to be vindicated on the 10,000 psi rise from a bullet touching the lands, but didn't really see how it could be any other way? I'll post on 10,000 psi being reached in the case before the bullet moves next. Work is just keeping me too tied up to sit and write at one stretch.
My copy says 2nd edition, printed in 2000. It is the only version you can buy new now, of course, but proper referencing requires the edition to be mentioned to avoid possible errors by persons looking in the 1998 edition. A search of Amazon.com turned up one copy of the 1998 edition, though I had first heard mention of it in comments by Dave Brennan, Precision Shooting's editor, referring to a possible follow-up book by Vaughn being in the works. I don't know what became of that project?
Vaughn measured 1200 lbs applied force in the static case. He then estimated that quantity would be halved in the kinetic case, to 600 lbs, which halved quantity, he stated, would take 10,000 psi to be produced on the .270 bullet. I believe, without looking back at it, that I quoted Vaughn directly on that point in my first response to Al's misreading of Vaughn's data. Doubling the force back to the static case would correspondingly double the required pressure to 20,000 lbs.
Al's explanation that he intentionally used units inconsistently, mixing psi and lb on purpose, is really not very satisfactory, but taking it at face value, then his error is in thinking a trained engineer and scientist like Vaughn would do the same thing. Vaughn did not. When Vaughn said he measured 1200 lbs in the static case, that's what he actually meant. Not 1200 psi. Otherwise he would not have subsequently referred to half that force, 600 lbs, requiring 10,000 psi acting on the bullet. 10,000 is, of course, a magnitude almost 17 times bigger than 600. So when Al says: "BTW all of this "factor if 17" stuff and "331,800 psi" stuff is just goofy-speak", it just shows, again, that he doesn't understand this relationship or the mathematical application of the units. I don't know how else to interpret that?
The other Vaughn statement Al referred to about pressure decline with additional seating depth (additional distance off the lands) applies only to one distance or another off the lands, and does not address the touching case. It is also true only over a limited range of distances off the lands. If a bullet is large enough and seated deeply enough, pressure starts to climb again due to reduction in combustion space at the start.
I am trying to get hold of a copy of "Reference 1", Absolute Chamber Pressure in Center Fire Rifles, the book whose numbers Vaughn himself believes on this topic. If I succeed, I'll report what it says?
P.S. Bullet 94,
Looks like I started composing and got interrupted and you posted the link in the meantime. Thanks for that. I thought of bringing up blow-by before, but it is not a terribly consistent behaving thing, varying with throat dimensions and style (ball vs. freebore) and the diameter of freebore where there is one, and, of course, bullet obturation, which is greater with the thin skinned match bullets than with solids, just to pick two extremes.
Glad to be vindicated on the 10,000 psi rise from a bullet touching the lands, but didn't really see how it could be any other way? I'll post on 10,000 psi being reached in the case before the bullet moves next. Work is just keeping me too tied up to sit and write at one stretch.
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Unclenick
On page 48 in the link I found the pressure difference between the bullet being seated at the cannelure and the bullet being seated in contact with the rifling is all most a straight line. Which would seem to indicate that the seating depth difference of .250 = 10,000 psi difference. If you divide both the pressure and the difference in seating depth by 10 it would seem that .025 of a change in seating depth would give a 1,000 psi difference. This seems to confirm what Al states. And when the bullet was in contact with the lands the pressure didn’t seem to rise anymore than when the bullet wasn’t in contact with the lands. I’m not an expert but it seems to me after viewing this data that the pressure increase has more to do with the amount of gas leakage than the bullet contacting the lands. Am I missing something?
On page 48 in the link I found the pressure difference between the bullet being seated at the cannelure and the bullet being seated in contact with the rifling is all most a straight line. Which would seem to indicate that the seating depth difference of .250 = 10,000 psi difference. If you divide both the pressure and the difference in seating depth by 10 it would seem that .025 of a change in seating depth would give a 1,000 psi difference. This seems to confirm what Al states. And when the bullet was in contact with the lands the pressure didn’t seem to rise anymore than when the bullet wasn’t in contact with the lands. I’m not an expert but it seems to me after viewing this data that the pressure increase has more to do with the amount of gas leakage than the bullet contacting the lands. Am I missing something?
Along with all the technical stuff already mentioned...
I just want to state that the loading manuals all have one thing in common, their data is worked up in test rifles. It may be a universal receiver, or it may be a specific rifle make and model, but all of them state that their loads were safe in their test guns, and should be approached with caution in your gun. This is not something to be ignored.
While the majority of guns behave very similarly, they are individuals, and some of them are way more indiviual than others. What is a safe combination of components and seating depth in one gun may not be safe in another. Never forget this simple fact. The data in reloading manuals is not gospel, it is guidelines, on both ends of the recommended safe loads. It is rare, but some guns will show pressure signs with "starting loads" (and still be in correct mechanical condition), while others will take loads over listed "max" without batting an eye.
All you can be certain of is that the listed loads worked in the guns they used to test them. There is a high probability they will work ok in your gun, but it is not an absolute certainty. Take it from there, with caution.
I just want to state that the loading manuals all have one thing in common, their data is worked up in test rifles. It may be a universal receiver, or it may be a specific rifle make and model, but all of them state that their loads were safe in their test guns, and should be approached with caution in your gun. This is not something to be ignored.
While the majority of guns behave very similarly, they are individuals, and some of them are way more indiviual than others. What is a safe combination of components and seating depth in one gun may not be safe in another. Never forget this simple fact. The data in reloading manuals is not gospel, it is guidelines, on both ends of the recommended safe loads. It is rare, but some guns will show pressure signs with "starting loads" (and still be in correct mechanical condition), while others will take loads over listed "max" without batting an eye.
All you can be certain of is that the listed loads worked in the guns they used to test them. There is a high probability they will work ok in your gun, but it is not an absolute certainty. Take it from there, with caution.