We’ve posted a couple of times about the interesting CMMG MK 47 Mutant, which mates an AR-style operating mechanism to an intermediate-length upper receiver and a lower receiver that takes AK magazines. The marriage of the AR-15’s direct impingement operating mechanism and the 7.62×39 round is an interesting one, and many may be curious how those two perform together in an extended firing session (such as a carbine course). Mrgunsngear has obliged us with an (attempted) 1,000 round-count firing session with the Mutant.

Unfortunately, near the 600 round mark (600 rounds in that session; it had previously had about 1,200 rounds through it) the rifle began having issues. Initially, my suspicion was the ammunition: The rifle was failing to extract, which is normally an ammunition issue, but that hypothesis was disproven as the rifle continued to fail to operate properly:

What was the issue? Mrgunsngear goes into detail below:

The rifle did not suffer from its ammunition (though it did not agree with every magazine tried, and Mrgunsngear was very thorough in trying a wide variety of mags), or its direct impingement mechanism, but the screw-retained gas block came loose and failed, causing the malfunctions. This problem shouldn’t concern most users of the rifle, but does highlight the importance of pinned gas blocks (or at least staked screws) on rifles that will see hard use.