The situation of shoulder set back causing the case headspace dimension to diminish is a common one, especially in rifles that are push feeds with plunger ejectors when cartridges having LR primers are used with true squib, cat's sneeze or mouse fart loads are used. The shoulder of the case is set back because the chamber shoulder “resizes” it back a bit on each firing. This is caused by the force of the firing pin driving the case forward so the shoulder hits the chamber shoulder. The primer explosion in the primer pocket also drives the case forward. Both those actions are what sizes the shoulder back a bit each time. How much is dependent on numerous things. With such loads the powder charge does not create enough psi to expand the case to grip the chamber walls at all. The primer explosion in the pocket may or may not force the primer back out of the pocket; usually not as the force of the firing pin spring on most bolt actions will hold it in the primer pocket.
Thus there may not be the “visual” of the primer backing out to tell us of the problem. Oiling or lubricating the case is tantamount to lubing the case for resizing; just makes it easier for the primer explosion to size the shoulder back.
In push feed actions this can lead to misfires after several firings as the case is pushed deeper into the chamber after each firing until the firing pin no longer has the reach to the primer for proper firing. Extraction of the case can also be a problem if the case is far enough into the chamber that the extractor can not engage the rim.
On controlled feed actions misfires and extraction seldom becomes a problem as the shoulder can be sized back only to the extent to where the extractor will hold the case back. On most CRF actions the firing pin is adjusted to have sufficient protrusion to fire a cartridge held by the extractor without the case shoulder “headspacing”. Actually that is the situation with factory ammo fired in most all CRF rifles, especially milsurp rifles. Seldom do the cases in such “headspace” on the case shoulder when fired.
It is with push feed actions that the real problem arises of the case should set back with truly light loads, especially cast bullet loads. With medium or normal cast bullet loads shoulder set on the case is seldom a problem. If large numbers of such squib, cat's sneeze or mouse fart loads are going to used repeatedly in the same cases the easiest way to avoid case shoulder setback is to simply drill the flash holes out. A #28 drill is the largest to use with LR primers. This prevents the force of the explosion, which considerable testing has shown, causes most of the case shoulder set back with such loads. It has the additional benefit of getting the flash into the case quicker and more uniformly resulting in better ignition uniformity and less “powder position sensitivity”.
Extensive testing the .308W and the 30-06 in CRF and PF actions with a cat's sneeze load of 3.2 gr Bullseye under the Lee TL314-90-SWC using cases with and w/o the flash holes drilled have consistently demonstrated the benefits of drilling the flash holes. Cases without the flash holes drilled developed case headspace issues in as little as 3 firings cause misfires in PF actions depending on the cases used (thickness of the brass and degree of anneal at the shoulder). Cases with the flash holes drilled were fired many times without any case headspace set back. Uniformity of ignition and internal ballistics were improved as was accuracy on target at 50 yards. As the psi was less than 7 ”€œ 9000 a pressure reading via the Oehler M43 was not obtained.
The cases with and with out the flash holes drilled were also tested with “normal” cast bullet loads of the 311291 over 4895 and a dacron filler in both the .308W and the 30-06 in the 1850 1900 fps range. No discernible difference in psi between the cases. There was only a slight improvement in the internal ballistics when the cases with the drilled flash holes were used. The ES and SD of the velocity and the psi were smaller and more uniform. The accuracy on target at 100 yards was also slightly better but additional and more thorough testing would be needed to determine if there is a real benefit with drilled flash hole cases with such medium or “normal” level cast bullet loads. Additionally equivalent loads duplicating M2 and M80 ballistics were tested in the cases with and without the flash holes drilled. Again there was no discernible difference in psi or velocity. The internals were remarkably similar. However, while I do recommend the drilling of flash holes for the use of squib, cat's sneeze or mouse fart loads I do not recommend for normal loads, cast or jacketed.
LMG
Concealment is not cover.........
