11-06-2023, 11:43 PM
SDguy Wrote:Why is a bolt action 6mm ARC load allowed to run higher pressure limits than a bolt action 6.5 Grendel load? Expect the same question will apply to 22 ARC vs 6.5 Grendel when the data is out.
The reason has to do with the bolt strength. The Grendel was essentially based on a variant of the 6mm PPC case with the shoulder blown foreword (one of the developers, Arne Brennan referred to it as the PPCX case). The PPC is typically run above 60Kpsi. The issue is that the Grendels developers decided to design around the AR15 platform with minimal modifications and not develop a different bolt and barrel extension that could deal with higher thrust levels.* The thrust against the bolt is the (max internal case dia) x (max chamber pressure). The bolt was designed for the 5.56x45 with a case dia of .376". The Grendel/ ARC case is .441" dia. That meant reducing the max chamber pressure to avoid bolt breakage. Since the Grendel was going to be limited to 52,000psi anyway, the original Lapua case was designed with relatively thin case walls near the base to maximize propellant volume. If the pressures get too high or extraction starts before the chamber pressure has dropped enough, the case bulges and you can get the "Dreaded Grendel Belted Magnum".
*Beefed up AR15 bolt/extensions do exist. CMMG uses the oversized"powerbolt" on the Resolute MK47, Colt did with the the CK901 in 7.62x39 for Yemen, Remington for the 30 Rem AR, DTech still makes them for WSSM AR15's.