Blowback 9mm Carbines

[zaitcev]

Why we need the correct reciprocating mass?

Too little reciprocating mass on blowback doesn’t allow adequate resistance (mass) to hold the detonating cartridge in battery so an out of battery (OOB) detonation can occur.

Too much reciprocating mass can cause “bolt bounce” where the momentum of the excess mass causes the bolt to chamber a round with enough residual force to bounce off the breech face and bounce. The disconnector in the trigger should prevent the sear to release but sometimes it is in just the right distance away an OOB detonation can occur.

AR9 OOB is a topic that's close to my heart and information like quoted above tends to trigger me. The main reason why AR9 is so prone to OOB is that it contains no positive OOB safety whatsoever. The disconnector has no such function!

In the AR-15, the parent design of AR9, the positive OOB safety is provided by the bolt carrier and bolt being two separate components. As the bolt carrier moves to unlock, it prevents the firing pin from protruding through the bolt face in a positive way.

A corollary of this is that 9mm guns that retain AR-15 design remain safe. For example, CMMG RDB guns, Macon DI guns, and Flint River piston-driven guns fall into that category.

This should explain why minimizing bounce alleviates OOB danger. The gun remains fundamentally unsafe. If the bolt does not stop completely in battery for any reason by the time the hammer falls, it will suffer an OOB detonation. However, if the bolt never stops just outside of battery, and you do not use something like "gat crank", the gun will not have a chance to fire out of battery.

Certain guns, such as Ruger PC Carbine and B&T GHM9, provide a measure of OOB safety with a flat rear of the bolt in conjunction with a flat face of the hammer. In this design, after a certain travel of the bolt, the hammer impinges upon the bottom of the bolt and cannot reach the firing pin anymore. If the barrel of such gun is of "fully supported" design, a certain amount of out of battery condition is not enough to rupture a 9mm case. Unfortunately, even when using a flat-faced mil-spec hammer, no AR9 bolt has a flat area that extends low enough to work as OOB safety.