There are several signs that your ammunition pressures are too high. Most of these are thanks to the considerable force of ignition distorting the case and primer adversely. Here are the most common issues:
Primer Cratering — The point of impact of the firing pin shows a raised lip indicating where the primer started to extrude around the pin into the firing pin hole. In extreme cases, this leads to…
Pierced Primers — The primer deforms so severely around the pin that the surface ruptures. In addition to possibly ejecting loose material, this causes the gasses in the cartridge to escape to the rear.
Case Head Flow — The brass of the case itself deforms under the ignition pressure, extruding into open cavities around the ejector pin area. This will cause small flecks of brass to shear off the case.
Brass Smear — Brass from the case deforms into any small negative space in the bolt face and deposits there. Some degree of this is natural, but high pressure loads will exacerbate the issue.
The results on these phenomena are reduced service life and reliability.
Pierced primers will erode the bolt face prematurely. Deformation of primers and case heads will lead to loose material in the receiver cavity. These metal shavings will eventually foul the ejector and can even impact function of the trigger group.
Shavings that fall into the magazine can also cause problems. Even a small piece of brass adhering to the exterior of a case can cause a failure to go into battery. One might also wedge the case into the chamber making it difficult to unload an unfired round.
If high-pressure ammo is so problematic, why is it so common? The short answer is that these problems are particular to gas guns. Yet, load development for these cartridges is almost exclusively done with manual rifles.
Ammunition manufacturers are not concerned or often even aware of the pressure issues that show up in semi-auto platforms. Amazing as it is, a particular round might function problem-free in a bolt gun yet be completely incompatible in a semi-auto.
Most ammunition manufacturers are concerned only with meeting SAAMI spec. in their load test fixtures. The compatibility of their ammo with gas-operated, semi-auto rifles was not even considered in many cases.
The major difference here is one of the bolt design. The standard .308 bolt uses a firing pin that is inherently too large and an ejector design that exacerbates the problems mentioned above. As avid competitive shooters, we at JP wanted a workable solution.
The only solution is a new bolt design, or should that be was…