A question came up asking what’s the best definition of a maximum load. It was in reference to reloading data stating the heaviest charge weights of different powders listed for a given bullet for a given cartridge. Several remarks about different data sources establishing their maximum were made and they’re quoted below.
Primer and powder lot differences will also change velocity and pressure; powder typically the least.
There is no SAAMI specification standard for bullet release force, or neck tension, for what’s needed to push the bullet out of the case neck. A given load with just enough case neck grip on the bullet to hold it may have a 2 pound release force; sometimes what benchrest competitors use for single round loading. Or one with a heavy crimp on a bullet that had .003" to .004" of interference fit in the case mouth that’s got 60 or more pounds of release force. A 30 caliber bullet with a 10 pound release force needs about 135 psi in the case to push it out. Same thing with a 60 pound release force needs about 800 psi to push it out. Force equals psi times cross section area of the bullet. A ..308" diameter bullet has a cross sectional area of .0745 square inches.
It takes a lot more chamber pressure to push a .308" diameter bullet into a 30 caliber barrel with a .307" groove diameter than a barrel with .309" groove diameter. If you check SAAMI specs for a .308 Winchester’s barrel’s bore and groove dimensions then compare them to bullet diameter specs, you’ll see a few thousandths spread in them. If the load was developed with a loose fit of bullet to barrel, it’ll have higher pressure and muzzle velocity in a barrel with a tight fit. The reverse also happens. So comparing ones data for their reloads against what’s listed in a manual can be skewed a lot by this issue alone.
Muzzle velocity, the ammo parameter’s that produces the largest spread across people testing a given load. Newton’s laws come into play and the more a barrel moves backwards while the bullet’s in it, the slower that bullet move over ground after it leaves the barrel. Rifles resting on something untouched by humans except for a finger on their trigger (benchrest free recoil set up and a 2 ounce trigger touched only by very light finger pressure) produce almost the lowest muzzle velocity and the smallest extreme spread and standard deviations. Fixed barrels in a SAAMI spec universal receiver in a fixed mount produce the highest muzzle velocity and lowest ES and SD numbers; they counter Newton’s laws and don’t move in recoil while the bullet goes through the barrel. Humans holding a rifle against their shoulder are somewhere in between and will have a wide range of muzzle velocity numbers across several holding the same rifle shooting the same ammo; a 100 fps spread in muzzle velocity can happen.
I asked:
Some loads are tested, but not with all the same equipment and methods. Rarely are their barrels at SAAMI specs for bore, groove and chamber dimensions. A barrel with smaller bore and groove diameters than what was used for the test will produce higher velocities and peak pressure with the actual load used for their test.
Primer and powder lot differences will also change velocity and pressure; powder typically the least.
There is no SAAMI specification standard for bullet release force, or neck tension, for what’s needed to push the bullet out of the case neck. A given load with just enough case neck grip on the bullet to hold it may have a 2 pound release force; sometimes what benchrest competitors use for single round loading. Or one with a heavy crimp on a bullet that had .003" to .004" of interference fit in the case mouth that’s got 60 or more pounds of release force. A 30 caliber bullet with a 10 pound release force needs about 135 psi in the case to push it out. Same thing with a 60 pound release force needs about 800 psi to push it out. Force equals psi times cross section area of the bullet. A ..308" diameter bullet has a cross sectional area of .0745 square inches.
It takes a lot more chamber pressure to push a .308" diameter bullet into a 30 caliber barrel with a .307" groove diameter than a barrel with .309" groove diameter. If you check SAAMI specs for a .308 Winchester’s barrel’s bore and groove dimensions then compare them to bullet diameter specs, you’ll see a few thousandths spread in them. If the load was developed with a loose fit of bullet to barrel, it’ll have higher pressure and muzzle velocity in a barrel with a tight fit. The reverse also happens. So comparing ones data for their reloads against what’s listed in a manual can be skewed a lot by this issue alone.
Muzzle velocity, the ammo parameter’s that produces the largest spread across people testing a given load. Newton’s laws come into play and the more a barrel moves backwards while the bullet’s in it, the slower that bullet move over ground after it leaves the barrel. Rifles resting on something untouched by humans except for a finger on their trigger (benchrest free recoil set up and a 2 ounce trigger touched only by very light finger pressure) produce almost the lowest muzzle velocity and the smallest extreme spread and standard deviations. Fixed barrels in a SAAMI spec universal receiver in a fixed mount produce the highest muzzle velocity and lowest ES and SD numbers; they counter Newton’s laws and don’t move in recoil while the bullet goes through the barrel. Humans holding a rifle against their shoulder are somewhere in between and will have a wide range of muzzle velocity numbers across several holding the same rifle shooting the same ammo; a 100 fps spread in muzzle velocity can happen.
More often than not, that’s caused by a full length sizing die being set too far down in the press. Rimless bottleneck cases get shorter when fired and lengthen when fired. The more the fired case head space differs from resized case head space, the shorter case life will be.
Most people are poor guessers for peak pressure. Having shown fired proof load cases to people along with normal safe load cases of the same make with the same primer, their guesses for peak pressure ranged from both equal and safe and one being max and the other a reduced load to both looked ok but one’s not quite as high as the other.
With all sorts of methods to establish average peak pressure, there’s easily several (sometimes huge) percent spread in what maximum is for a given cartridge, powder, bullet and primer.
True, but everyone does not use SAAMI spec tools, procedures and standards to determine max pressure for a given load. A universal receiver (Ulysses Machine Company referenced in current SAAMI specs, http://www.ulyssesmachine.com/, when mounted in a fixed position, it eliminates muzzle velocity inconsistencies caused by humans hand holding rifles against them) holds a SAAMI spec test barrel. One for the .308 Win can be seen showing detailed dimensions on page 306 in http://saami.org/specifications_and_information/publications/download/206.pdf. Other cartridges are shown in that same section. What powder and bullet making companies use SAAMI spec stuff to qualify their published data?
I asked:
No. Not when tested using SAAMI spec stuff to test it. Especially if it was developed with a cartridge with different component assembly details and shot in a barrel with much larger bore and groove diameters. If the load was developed with a barrel having large bore and groove diameters and low bullet release force, it’ll have much higher peak pressure and muzzle velocity if shot in a barrel at the small end of bore and groove specs.
Anyone have proof this happens? Never heard of such a situation. As long as the barrel and action steel’s elasticity limit’s not exceeded, I don’t think they’ll ever weaken. But the barrel will erode away at its breech end from being shot many hundreds or thousands of rounds depending on peak pressure and charge weights.
Yes, they should be, but they’re not all that way. Especially those developed by estimating peak pressure from case and primer deformations. There is no standard as to how flat a fired primer of a given make and type must be to indicate a given peak pressure in the case. And a given load in a given rifle will show different peak pressures measured by different people with different strain gauge systems depending on how they calibrate it and set it up. That’s compounded if their barrel’s different in bore and groove diameters and lengths than what was used to develop the listed data.
Good idea. But all loads judged by primer and case appearance after firing will not be within SAAMI parameters. That’s so often been proved a poor way to “measure” average peak pressure. Way too many variables that not many folks are totally aware of.
