Are speed and pressure directly related?

[MightyMO1911]

When working up loads, can we assume that once we hit the advertised speed are at least relatively close to the advertised pressure? And along that, can we continue working up above max listed charge if that's what is needed to attain that speed? I'm just trying to understand how pressure and speed correlate.

 

[skizzums]

absolutely NOT!!

are you trying to make a hunting round? is this for rifle or pistol? and what is it that you are trying to achieve by focusing on velocity as a key component?

how does it correlate? well, it does, but not in any calculatable way to indicate if your surpassing your max pressures. do you understand how to visually diagnose over pressure signs?

I am not being rude, I am just trying to understand your level of knowledge on how to read warning signs when reloading
the only reason I can think of to reload to velocity is if you are using a hunting bullet that is advertised to work only after a certain speed, or if you are making super long range ammo that you need to keep supersonic to a certain point. what are you tring to do with ths round and what caliber are we talking?
if your talking pistol, you can dangerously overload your charge if going for an advertised velocity of "X" from a 10" test barrel out of a 4" glock. your just not going to get there with your gun with possibly blowing off a finger or three.

max pressure is max pressure regardless of the velocity. some guns could get 300 more FPS while creating the same pressure that your gun is making 300fps less.

 

[HiBC]

Sort of.Pretty much.

The ammo mfgr's use their test equiptment and blend "adjustments" into the loads they produce to compensate for batch tolerances.

A handloader has limited resources.

Yes,if book max is 50 gr of IMR 4350 to achieve 2800 fps,and you load 47 gr and chrono 2800 fps,it is prudent to assume you have reached max pressure at max velocity,even though your powder charge appears to be light.

I did say "prudent".We can't know what your pressure is.The velocity is a good indicator.

At the same time,a sticky bolt handle is worthy of attention at 47 gr.Even if you are only getting 2730 fps.

There are variables,from your scale calibration,your bore dia,twist,lot to lot variation,etc.

So,yeah.Velocity and pressure go hand in hand.

 

[BigJimP]

No, velocity and pressure are not directly related on a linear graph....../ to assume they are, is not smart in my view. Yes, there is a correlation between them ...but predicting either below or above published data is not predictable.

......and every powder is different...some pressures are listed in PSI ..some in CUP ..and there is no direct conversion from one to the other either...

To load anything above published maximum in the tables is foolish in my view - ( ....but its your hands and eyes ...and your guns...)...!

If you need something faster than one powder bullet combination will give you ...there are plenty of other powders out there, with published data, that will give you what you need ! No reason to be an amateur chemist..

 

[MightyMO1911]

Skizzums I'm not looking to achieve any certain speed and yes, I have a basic understanding of observing pressure signs. I have a lot to learn in that area, which frankly is the main purpose of this thread. A learning experience. One thought is if my load is higher than the advertised speed, can I assume I'm over max pressure, even though there may be no obvious signs? I'm just expanding my base knowledge of load work up.

 

[Hidalgo1]

Read what Big Jim wrote above.

Learn to read pressure signs.

I'm certain that it happens, but I can't remember ever going over published speeds with powder charges that were less than published max.

 

[skizzums]

no, if your speed is higher than advertised, it could mean that yur barel is longer for one, your barrel can be tighter in tolerance. different primer, different brass can make small differences. twist rate can make a difference and powder lot. I would never base my max load on velocity. generally you can work up by small increments until you start seeing(I don't know if your loading for pistol or rifle) flattened primers, abnormal firing pin impressions, exaggerated extractor marks, sticky bolt is a big one or any difficulty in extraction that is abnormal when shooting factory loads. how far is your brass ejecting in pistols(not always relevant, bu can help), goofed up rims, "cratered primers(or excess material around the pin strike, kind of looks like a volcano shape). bent rims and of course, pierced primers. I can show you some pics if you tell me what your loading for that have some telltale over pressure signs.

anyway, you can load until you see ONE of these symptoms and then back off a grain or two and say that's your max. but for pistol, its almost never prudent or reasonable to load to max unless you are hunting with it. rifles are different, have to surpass max with .223 to shoot 600yards efficiently, but max is almost never most accurate in pistol loadings

 

[44 AMP]

can we assume that once we hit the advertised speed are at least relatively close to the advertised pressure?

Turn that thinking around and you'll be closer to reality. Once you hit the specified pressure you should be close to the advertised speed.

Don't overthink it. Pressure is what pushes on the bullet. What speed the bullet gets from that pressure is dependent on the pressure AND other factors.

In theory, every bullet of the same weight fired with the same pressure should have the same velocity. But reality is different. Two identical appearing guns firing the exact same ammo can have velocity spreads of as much as 100fps, although much smaller variations are much more common.

Design of the gun, length of barrel, fit of the bullet in the barrel, friction of the bullet in the barrel (bearing surface of the bullet, and hardness of same), even how the shooter holds the gun during firing all make a measureable difference in the actual velocity obtained. Note that "measureable" is not automatically the same as "practical".

When you get to max safe working pressure (determines either by reaching the published max load level, OR by pressure signs AT ANY TIME, no matter what the load level, you are there. Whatever velocity you got is what you get, from your gun and your components. It should be relatively close to what the published data is, but can be much different. No matter what it is, its what you are going to live with.

 

[Jimro]

Peak pressure is the highest pressure point during the firing cycle. If you graph pressure over time, you'll see it starts at atmospheric pressure (which we'll call the zero or baseline), builds up to peak pressure, then tapers off.

The area under the curve is that is going to be more easily related to velocity, as it will represent the sum total amount of pressure that pushed on the bullet during the firing cycle.

Different powders have different burn rates and therefore they have different pressure curves, some building more slowly, some building more quickly.

Now it would be easy to assume at this point that simply choosing the powder with the most area under the curve would give you the most velocity, but that would only be true if this were a static system. The volume of gas has to fill a constantly expanding area as the bullet goes down the bore, so where the bulk of the area under the curve falls in relation to timing in the firing cycle is also very important.

Suffice to say, it is much easier to look at the data in a reloading manual and pick out a suitable powder that provide the velocity range you are seeking than to try to do integral calculus on pressure curves...

Jimro

 

[mehavey]

Within the realm of reasonable differences for similar barrel lengths for the same powder/projectile, pressure tends to vary as the cube of the velocity ratio.

Predicted Velocity: 2,800fps
Predicted Pressure: 50,000psi

Actual/Measured Velocity: 2,900fps
Likely Pressure ~ 50,000*(2900/2800)^3 = 55,600psi

 

[jmr40]

Lets say a particular load listed in a manual shows 47 gr of powder will give 2900 fps and you should be @ 59,000 psi. And if that exact load chronographs 2950 fps in your rifle then you are probably over 59,000 PSI and that load is a little too hot in your rifle. When you reach 2900 fps, you are at the max pressure, not necessarily the 47 gr listed in the manual. In this case a max load is somewhere less than the listed 47 gr of powder.

On the other hand, that exact load may only chronograph 2800 fps in a different rifle, and be well under 59,000 PSI. You could probably load more than the 47 gr of powder to get the speed closer to 2900 fps and it would be safe in the 2nd rifle. I STRONGLY advise against doing so. If the loads that have more than 47 gr of powder find their way into the 1st rifle, then you have a potentially dangerous situation.

 

[cdoc42]

Maybe this will be helpful. When I'm looking for a new load to workup, I'll look for the maximum load that gives the highest velocity at the lowest pressure as a starting point.

For example, the Hodgdon Reloading Data Center lists 7 powders for the .270 Win Short Mag for the 150gr Hornady Spire Point. Of those, pressures at maximum load range from 61,800 PSI for Hybrid V100 delivering 2954fps to 63,200 PSI for H414 delivering 2991fps. It is not worth gaining 37fps with the increase of 1400PSI.

The list also shows H1000 delivering 3001fps at 62,800 PSI, so I have no reason to use the powder having the greatest pressure at a lower velocity (H414). It MIGHT produce the best accuracy at any given velocity, but it is last on my list of choices just because of compared pressures.

Let's say I choose HybridV100. Start load is 54.0 grains, max is 59.0. Seasoned advice is to start with 10% less than listed, but I have never had excess pressure signs by starting in the middle (56.5gr - which I would round off to 56.0gr) and stop at 1 gr under the max, which is 58.0gr. I load 3 rounds each at 56, 57 and 58gr. Let's say this gives me a 1 MOA group (lucky day) with 57gr. I'll then try 3 with 56.5 and 58.5. In MOST cases I never have to get to 58gr to achieve the best accuracy, which is really what I'm looking for, whether it be hunting or target practice. If my best accuracy is at 58.5gr but I'm seeing signs of excess pressure, in this example I would move to H1000 and start all over.

A word of caution. This method assumes you are using the Hornady 150gr Spire Point. But since I am under the max listed load, I feel comfortable doing the same thing with any 150gr bullet in .270 caliber. Just watch for pressure signs.

Also be wary of comparing pressures with different powders. In the Hodgdon site you can ask to view IMR powders WITH the Hodgdon powders. The IMR powders may list CUP rather than PSI, and these are not the same. But you can still choose an IMR powder if you compare more than one the same way I described choosing the Hodgdon powders.

 

[steve4102]

When working up loads, can we assume that once we hit the advertised speed are at least relatively close to the advertised pressure?

And along that, can we continue working up above max listed charge if that's what is needed to attain that speed?

I'm just trying to understand how pressure and speed correlate.

Rule Number (1), Never exceed Max loads.

My rule Number (2), Once published ( more than two sources) Velocity is reached, you are done, even if Max powder charge has not been reached.

 

[g.willikers]

Well, then we're in agreement that the answer to the question is yes, no and maybe.
Other than entertaining our chronographs, or trying to improve those long shots in some magical way with increased speed,
What is the motivation of loading maximum pressure rounds, in the first place?
Since without the necessary equipment, we can never really know what the pressures of reloads truly are,
Why even attempt to make a "take it to the Max" loads?
It's hardly required to hit even long range targets.

 

[HiBC]

I certainly agree with reading all the indicators for pressure.
They are all tools to be applied with prudence.

While we all can arrange our priorities as we choose,I think,within responsible,safe limits,there is nothing wrong with pursuing maximum performance..maximum safe velocity.

Forget the advertised factory load max velocity.To a degree,its marketing propaganda,and also the ammo factories have resources that we reloaders do not have.

Consult at least two manuals.If they disagree,find a third manual or go with the conservative load.

Do begin with the starting load and work up,observing for the common pressure signs.
To say it another way,do not assume you will achieve max published velocity.If you get cratered primers and a sticky bolt 100 fps short of published max velocity,accept that you hit the wall,and back off a bit.

Now,the way I read the OP's question,written another way,"As I begin from the starting load,and work my way up,say,toward a published max load of 50 gr powder for 2800 fps,
If I achieve 2800 fps at 48 gr of powder,shall I take that as an indicator that I have reached maximum acceptable pressure,as I have achieved maximum published velocity?

Bolt opens easy,primers look good,if you want to mic case heads,allgood..no other pressure signs.

Yes,OP,I do think it wise to assume hitting max velocity is an indicator you have achieved max pressure,even though the book gives you two more grains.

The point is to look for a number of reasons to say "Enough".Do not ignore any of them.

A GPS is a pretty good,reliable navigation tool.But,beware that if a bridge washes out,your GPS may advise you to proceed at your own peril.
Your eyes tell you "Bridge is gone" Don't keep following the GPS.One more little thing.
Not all handloads are fire in top tier strength firearms.The recognized "pressure signs" are about brass and primer behavior in modern bolt action or equivalent firearms.
It is not a good idea to load an original 30-40 Krag till you run into "pressure signs" You may find that single glass hard locking lug broke off,and that is your pressure sign,but,dang,those primers look fine!!
Suppose I am loading 168 gr bullets with H-4895 to feed a Garand.It might be that with lot "A" of 4895 I loaded 46.3 gr to get 2630 fps.Shoots good,I'mhappy,and it is easy on the rifle.I have notes,experience with 2630 fps.I get lot "B" of powder,or,even I can't get H-4895 and I have to use 4064.OK.If I can safely load to 2630 fps..Why wouldn't I choose to load to the chronograph?I then still use "x" number of clicks on the sights.

And,regardless of what any book load tells me,if I chrono a 200 gr bullet at 2600 fps out of a Krag or a 168 gr bullet at 2950 out of a Garand,I'm going to NOT SHOOT ANOTHER one.
Even if the brass looks good.

OP,I do think you asked a reasonable question .I hope I answered it for you.

 

[MightyMO1911]

. I'm not looking to go over max speed or max pressure. But since I have no machine capable of measuring the pressure I'm trying to learn the correlation of pressure and speed.

 

[RKG]

The discussion so far conflates, and in my opinion confuses, a number of discrete questions.

1) Are pressure and peak combustion pressure related? For one firearm, yes. However, if PP X produces MV Y in one rifle, same MV in a different rifle could imply different -- either higher or lower--PP.

2) Is that relationship linear? No. In real life, the curve over a wide range of charges, is polynomial, though the section within an operating range may appear nearly linear.

3) Outside of that operating range, again assuming one firearm, assuming all other factors held equal, and assuming a decent jump to the rifling, the curve of PP vs. MV rises sharply at the high end and, usually, drops off at a higher slope at the low end.

4) To complicate matters further, MV is not produced by peak combustion chamber, but rather by the integral of chamber pressure over the entire interval of bullet travel within the bore. This is why, for instance, with higher weight bullets, one can achieve higher MVs yet lower PPs by using a powder with a slow burn rate (and, perhaps, a greater degree of progressivity).

5) Likewise, even with the same firearm and load, COAL can have a measurable effect on PP. Reducing jump to zero for an otherwise acceptable load can raise PP way over nominally safe levels, yet yield no significant increase in MV. This is why bullet weight by itself does not imply load acceptability; not all bullets of the same caliber and weight have identical shapes, and bullet shape factors into resulting jump value.

6) In the main, analyzing and quantifying the relationship between PP (or integrated chamber pressure) and muzzle velocity is beyond the capacity--tools and skills--of amateur reloaders. Which is why one relies on reloading manuals.

 

[HiBC]

You are doing fine,and your thinking is reasonable,with one caution.Please just use velocity as another yellow/red light,and not a green light.

There is a process I use of watching for another indicator of "max".I'm not suggesting it,I'm just pointing out a pressure trend.

I record the relationship between an increase in charge,incrementlly,and an increase in velocity.If,frm starting load,I get a velocity increase of 45 fps for 1/2 gr of powder,it will remain fairly linear at 45 fps as I increase the charge...to a point.There comes an indicator of "max" if I get fewer fps,say 20 fps for the 1/2 gr.
The way my simple mind sees it,when the 1/2 gr only gets me 20 fps,the rest of the powder went to a peak pressure spike,rather than accelerating the bullet.
I do not blindly rely on this method.I use it as another yellow/red light,to go along with all other observations.

Pressure increases ,after a certain point,are not linear,they spike.

At some point the powder's production of gas,and pressure,outruns the bullet's acceleration.More accurately,it outruns the increase in combustion chamber/bore volume.
A small increase in velocity can make a great increase in pressure.

 

[jim8115]

Just be careful chasing velocity. If the max published load doesnt give the velocity you expect- stop- dont keep upping the charge to obtain what you think it should get.
I once had 2 "identical" model 28's. With a full 357 mag load, one ran 300 FPS slower than the other ( never did figure out why ). Had i been using the slow one, and attempted to get the velocity I expected by going over max, I probably would have destroyed the gun before i got there....

JIM

 

[Bart B.]

What would you do if powder A shot the bullet 50 fps faster than load B, but powder B was twice as accurate; shot groups half the size of A's groups?