A week or so back was a thread about Gordons reloading tool and it's accuracy. I did some research on using the tool and not surprisingly you get out of it what you put into it. I weighed my rifle, contacted the manufacturer of the barrel and got it's true twist, length, and gas port diameter. I had to use a 120 SMK because the Barnes 120 MB was not in the database but over all the computed Optimum Barrel time was off by .02 gns to what I found in real life. GRT precited 28.72 and in real life I found my node at 28.7 gns of TAC. Apologies about the image size of pic 1 but I use a large monitor and it did not reduce well. I reduced it to 1920 wide, any smaller and the text quality suffered. Pic 1 is the GRT screenshot, pic 2 is of the Shot Marker target measured in Ontarget software. Chrono readings on the Ontarget picture were transferred from my ProChrono which was appx 12 feet from the muzzle
GRT, OBT and load workups
- Thread starter hounddawg
- Start date
@Bart - iGRT calls it bullet resistance. You can also set the amount of the bullet that is gripped by the neck, and jump to lands
@Nick - one of the things that really pulled everything into line for me was setting the gas port diameter and location
although the OBT was way off I did find a lower accuracy node and GRT came really close to the predicted speed. Seems as if the OBT part is hit or miss but GRT if set up right is pretty darn accurate
@Nick - one of the things that really pulled everything into line for me was setting the gas port diameter and location
although the OBT was way off I did find a lower accuracy node and GRT came really close to the predicted speed. Seems as if the OBT part is hit or miss but GRT if set up right is pretty darn accurate
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It's getting it set up and all the right arguments in place. It's also being aware of things .k,e the Siebert factor default is for full pressure loads, and it changes with load size and pressure, but the defaults won't. Also, be aware, especially with small cases, there are circumstances when primers will unseat bullets before the powder really ignites, changing the effective seating depth. Lots of little details, but if you play with it enough, they come to make sense as you go along.
@ Nick. As far as FPS per grains of powder it seems to track really well, once everything is set up and calibrated.
I just got in 500 cheap 123 gn HPBT's from Mid South for practice. I am going to do a quick run up with LT 32 or AR Comp and do up a worksheet and see if I can get them to shoot decently just for fun/practice. I am just going to fire one at each increment for 10 increments and use my Magnetospeed sporter to get a true muzzle speed to calibrate the program and go from there.
I just got in 500 cheap 123 gn HPBT's from Mid South for practice. I am going to do a quick run up with LT 32 or AR Comp and do up a worksheet and see if I can get them to shoot decently just for fun/practice. I am just going to fire one at each increment for 10 increments and use my Magnetospeed sporter to get a true muzzle speed to calibrate the program and go from there.
Unkl Chuck
New member
Interesting thread, I just started working with GRT and am climbing up the learning curve at this time. Is there a parameter for primers? I haven't noticed one and primers can have some effect on pressures and/or bullet unseating.
hounddawg, your pix looked fine. What software are you using for your resize? I use some that predict output file size, but hesitate to recommend without knowing what you are using. Photography is my other principal vice.
hounddawg, your pix looked fine. What software are you using for your resize? I use some that predict output file size, but hesitate to recommend without knowing what you are using. Photography is my other principal vice.
@ Unkl Chuck A old version of paint shop pro from 2013. Never found a reason to upgrade since stuff like this is about all I use it for
on the primers
https://grtools.de/doku.php?id=en:faq:faq-primer
on the primers
https://grtools.de/doku.php?id=en:faq:faq-primer
Unkl Chuck
New member
Thanks hounddawg, I suspected that that was the final answer from GRT on primers. When I get to field testing, I'll stick to the primer recommended in the manuals. I'm still just doing exercises learn the program. Later we can play with primers.
If you can score a copy of Adobe elements, you might find it a little more user friendly. I've used PSP and now Corel Paintshop. My goto for file resize is Nikon Studio [the old Nikon View], a bundled software with Nikon digitals. Photoshop works great as well but it is big and expensive. You might look at Gimp, a free program.
If you can score a copy of Adobe elements, you might find it a little more user friendly. I've used PSP and now Corel Paintshop. My goto for file resize is Nikon Studio [the old Nikon View], a bundled software with Nikon digitals. Photoshop works great as well but it is big and expensive. You might look at Gimp, a free program.
thanks for the suggestions UC, back when I worked for a living I used PS and the old Corel Draw a lot, I also had a early version of GIMP on my Linux machine. Twenty or twenty five years ago I actually won a few graphics contests on the gaming sites but those days are gone. Now all I do with PSP is screen grabs, resize, crop, and compress these days.
back to GRT I am still in the early learning stages myself. I can think of a dozen things I want to experiment in the program as time permits.
back to GRT I am still in the early learning stages myself. I can think of a dozen things I want to experiment in the program as time permits.
You don't need separate software to resize images you post here. Just use the resize tags supported by the board software. Whether you host the image here or somewhere else, once you have the image url inside the image tags, you can put the size tags around it and give the desired pixel width. The height will automatically be resized proportionally.
For example, this will show the image in its native size:
[img]image-url-here[/img]
But this will display it 800 pixels wide:
[resize=800][img]image-url-here[/img][/resize]
For example, this will show the image in its native size:
[img]image-url-here[/img]
But this will display it 800 pixels wide:
[resize=800][img]image-url-here[/img][/resize]
You can read about it here. The idea is that pressure waves set up in steel reflect back and forth along a barrel at the speed of sound in steel (about Mach 17 as compared to air). This means they will make several transversals while a bullet is traveling down the tube. However, there are dead spots in the wave, and the theory is that you want the bullet to exit when the muzzle is located in the wave's dead spot.
As far as I know, the theory fails to explain why barrel tuners work, as adding a tuner doesn't affect the speed of sound in a barrel, whereas muzzle swing (commonly called barrel harmonics) does. It also doesn't explain velocity flat spots encountered when incrementing powder charges, which are the other explanation (in addition to muzzle swing) for why Audette ladders work. It may be that changing barrel time partly explains why tuning seating depth can tighten groups, but it is unclear. Nonetheless, lots of people seem to get good results tuning loads to OBTs. So it is tempting to think there may be four separate tuning factors in finding the most accurate load. However, all those things interact. You can't change seating depth without affecting peak pressure and barrel time. Barrel time variation affects timing coordination with muzzle swing. Change or lack of change in velocity determines barrel time. So, how do you isolate this stuff? How much interdependency is there? Does a barrel drilling bugholes have all these things working in unison?
As far as I know, the theory fails to explain why barrel tuners work, as adding a tuner doesn't affect the speed of sound in a barrel, whereas muzzle swing (commonly called barrel harmonics) does. It also doesn't explain velocity flat spots encountered when incrementing powder charges, which are the other explanation (in addition to muzzle swing) for why Audette ladders work. It may be that changing barrel time partly explains why tuning seating depth can tighten groups, but it is unclear. Nonetheless, lots of people seem to get good results tuning loads to OBTs. So it is tempting to think there may be four separate tuning factors in finding the most accurate load. However, all those things interact. You can't change seating depth without affecting peak pressure and barrel time. Barrel time variation affects timing coordination with muzzle swing. Change or lack of change in velocity determines barrel time. So, how do you isolate this stuff? How much interdependency is there? Does a barrel drilling bugholes have all these things working in unison?
That would probably mean the timing of the dead spots in the pressure wave would optimally be back from the muzzle half an inch. If the bullet was exiting at, say 2600 fps, then the last half an inch would be about 0.016 milliseconds early, so you would subtract that much time from the OBT that a 24" barrel normally has.
Some years ago I helped a fellow with a 270 Win sporter tune it by this method. He wound up with 5-shot groups going into about 0.3 moa at 100 yards. So it can work. I just can't say it works every time, and I don't have enough data on successes vs failure to say for sure what portion is attributable to the theory and how much to blind dumb luck.
Some years ago I helped a fellow with a 270 Win sporter tune it by this method. He wound up with 5-shot groups going into about 0.3 moa at 100 yards. So it can work. I just can't say it works every time, and I don't have enough data on successes vs failure to say for sure what portion is attributable to the theory and how much to blind dumb luck.