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The Sig MCX Multi Caliber Carbine: A Proper Way to Design an External Piston AR

By Christopher R. Bartocci

The trend of external piston operated AR-type rifles seems to be in a descending period. They do not appear to have the flair they did 10 years ago. The AR-type rifle was not designed for this operating system. It was designed as an in-line construction, direct gas impingement rifle. The main difference is how the gas is introduced. The AR-family was designed to have the gas directed into the bolt carrier where the piston chamber is made up between the back of the bolt and back of the carrier giving it a perfectly centered drive rearward. When utilizing an external piston the bottom of the rear location of the carrier is tilted downward when it enters the receiver extension due to the off center strike of the operating rod on the top of the bolt carrier, creating the common “carrier tilt” issues. Other issues include the receiver damage of the cam locking cut inside the upper receiver where the cam pin cuts into the receiver. The end result is adapting a weapon, which is not designed for that operating system, and the designer must develop solutions to overcome these new problems. If you want an external piston gun, why not design it that way?? The first true reconciliation of this problem was Robert Faxon with his ARAK-21. A complete redesign of the upper receiver that will fit on an AR lower receiver that eliminates all the problems of converting an AR upper to the wrong operating system! The receiver is a long stroke piston which has a bolt carrier that rides in rails in the upper receiver keeping it properly aligned so there are no carrier tilt issues, utilizing an internal action spring while maintaining the strong multi-locking lug bolt.

In 2012, Sig Sauer was approached by an undisclosed special operations unit with a requirement for a 300 Blackout rifle that would function with both sub and supersonic ammunition as well as when suppressed. According to Chris Sirois, then Design/Engineering lead on the program...


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