05-13-2022, 12:31 PM
Fess Wrote:Bill Alexander had stated that the SAAMI pressure max was set to prevent bolt breakage in an AR15 with minimal modifications. Bolt backforce is what must be resisted, and the worst case of that is the "piston" of the area of the case head pressing backwards at full chamber pressure. The larger the case head, the more backforce. The case head of the Grendel is larger than that of the 5.56x45, so it has to operate at a lower pressure to prevent breakage.
With the 6.5 Grendel, the bolt and corresponding barrel extension from the AR15 family was not enlarged from its 5.56x45 predecessor. That was a deliberate choice to make the conversion to 6.5 Grendel as simple and inexpensive as possible. It is not that it could not be done, for example, Olympic Arms briefly made AR15's that shot the short, fat .223 and .243 OSSM cartridges (variants of the WSSM line) for example. CMMG currently uses a larger bolt and barrel extension for its Grendel and 7.62x39 rifles and could likely handle more pressure as well.
Although it is possible to use exotic materials like maraging steel (as used in SIG's new 6.8x51 rifle) to make the current-sized bolt stronger, the 65 Grendel case itself can become the limiting factor as pressure is increased. The case was designed around the 52,000psi SAAMI pressure limit. The case walls near the base were kept relatively thin to maximize powder volume. This is generally not a problem for bolt action rifles since they often provide chamber support all the way to the case head. In autoloaders, however, the situation is different since they violently yank the case out before all the pressure in the barrel has subsided. The chambers also tend to be looser, especially near the entrance, to enhance feeding and reduce jamming problems.
Fess:
Thanks for the info. Folks need to remember the intent behind the Grendel which was more tactical than target.
When Oly Arms was still operational I lived close by and they gave me a converted 5.56 bolt that was actually pretty durable. Better than Colt 7.62 X 39 bolts (I broke two of them), and lasted as long as one of the AA / CSS 7.62 X 39 bolts and one AA Grendel bolt. Yes, they were making some really fat and short cartridges that actually functioned through the AR. I currently have a M-4 they made that I have shot a gigantic amount of service ball and handloads out of with absolutely no issues.
LR-55