The SIG Sauer MCX is a semi-automatic rifle similar to the AR-15. It utilizes a short stroke gas piston system that is designed to keep recoil to a minimum and reliability to the maximum. The design is technically based off of the pistol caliber MPX. The MCX is available for the 5.56 and .300 AAC, and is compatible with STANAG magazines. The upper receiver is also compatible with the lower receivers of the AR-15.