Originally Posted by
bronko22000
Thanks Nomercy. One more question now that I know I will have no issues with it in my Axis. But how about my AR? Will the slightly higher pressure effect my cycling much like using a hotter load? It seems that the designers used a differently shaped bullet for the reason you mentioned earlier to reduce the bearing surface. The Speer Hot Cors I am using are very similar to the ones in the factory ammo. I would imagine that using a standard .357 pistol bullet would alter the cycle pressure too causing the buffer/spring to take more of a pounding than necessary.
Not at all - AR’s aren’t blowback operated, so max pressure seldom matters for reciprocating speed - only port pressure and flow.
We’re not targeting a narrow +/-50psi pressure band for port pressure - we have the luxury of a wide tolerance for port pressure and dwell time. The case is small, and the pressure curve drops off relatively swiftly. Even running a pistol length gas system, the “throttling” of gas by the gas port diameter significantly limits the operating pressure exposed to the carrier.
The difference between two bullets in this case will yield FAR less difference in port pressure than changing bullet weight or changing powder type, for that matter.
So when we use a revolver type bullet, or a copper solid with a long bearing surface (high bore drag), we end up hitting our max pressure earlier in load workup, meaning a lower charge weight hits the same max pressure - which invariably reduces our port pressure later down the bore.
You can actually see the counterintuitive response in several cases - another example is running a fast powder which will hit a high pressure max early, aka, lower charge weight, earlier than a slower powder which would yield a higher port pressure after the same max peak. So we consider that charge weight as “max”, but then the charge weight is low enough such the pressure won’t be sufficiently sustained downbore, and the curve has subsided too much by the time it reaches the port and it doesn’t have the oomph left to cycle the action.