The CMMG Mk47 Mutant
By Christopher R. Bartocci
The 7.62x39mm cartridge is by no means new to the AR platform – clearly going back to the 1980s. Traditionally, all that was changed was the bolt, barrel and magazine. The bolt was a modified 5.56mm bolt to accept the larger 7.62x39mm cartridge base and the magazine was modified to a curved configuration to go along with how the cartridges stack due to the taper on the cartridge case. Since that introduction, rifles and conversions into the 7.62x39mm caliber have been sold by Colt, ArmaLite, Rock River Arms, Olympic Arms, PWS, YHM and Bushmaster to name just a few. With the increased popularity of AR-15 rifles, many now custom build their own using a variety of components.
There have been two major problems that have plagued this AR-based 7.62x39mm rifle. First is the magazine and second is the bolt itself. The taper on the 7.62x39mm cartridge case is excessive causing the curved AK magazine. The “banana” magazine is not made for looks but the curvature is an accurate representation of the cartridge stack of 15 rounds. The magazine well of the AR-15 is straight and since the 7.62x39mm cartridges are tapered the rounds do not sit in their natural position. When high capacity (30-round) magazines are made the top part is straight and then curves. There is no natural feeding in the magazine which has, and still causes, reliability issues with the rifle. This has plagued every rifle made over the years.
The next major problem is the strength of the bolt. In order to chamber the 7.62x39mm cartridge, additional material had to be removed from the breech face area drastically reducing the wall thickness of the supporting area of the bolt. Improving this has been quite difficult. Some manufacturers have gone to stronger materials for this bolt. Due to interchangeability, it was not an issue to increase the overall diameter of the bolt.
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