While aesthetically similar to and just as lethal as an AR-15, the MCX is internally a different beast, thus all but removing it from the AR-15 family of rifles. Yet while the weapon is different, the MCX and the AR-15 share the same design purpose: providing a highly portable, customizable, easy to operate and accurate rifle for the individual who possesses it.

As Bob Owens from the blog “Bearing Arms” points out, the MCX is a modular rifle designed to be able to change between a variety of calibers and “otherwise has no major parts that interface with AR-15s in any way, shape or form.”

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Originally designed for U.S. Special Operations forces, the MCX was built from the “ground up” to be lightweight and short, and it accepts “a broad array of accessories, enabling you to build a complete weapon system for any scenario or environment,” Sig Sauer’s website says.

While able to shoot the same caliber ammunition — .223 — as an AR-15, the MCX was initially meant to fire a round called a .300 Blackout. The relatively new caliber, according to Owens, was designed to provide Special Operations Command with a bullet that was as quiet as a pistol round but packed the range and lethality of a rifle cartridge. A side effect of the bullet’s design is that the it mimics the size of the round fired by AK-47-type rifles.

The key difference, however, between the standard AR-15 series of rifles and the MCX is the operating system used to mechanically propel the bullet from the gun and cycle the next round to be fired. The initial AR-15 (AR standing for Armalite Rifle), designed by Eugene Stoner in the late 1950s, used a system called “direct impingement.” Most modern AR-15-type rifles use this system.

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Direct impingement means that when a round is fired, the gas generated from the fired bullet is shot through a hole in the barrel and is then channeled back through a tube that leads directly to the weapon’s bolt assembly. The gas, proceeding back to the rear of the rifle, moves the bolt assembly to the rear, subsequently ejecting the spent shell casing. A spring in the stock of the weapon then propels the bolt assembly forward again, inserting another bullet from the magazine into the rifle’s chamber, thus restarting the whole process.

The MCX, however, is known as a “piston gun,” meaning it uses gas piston technology to operate the internals of the rifle. When the rifle is fired, instead of the excess gas going into a small tube that is directed to the bolt assembly, the gas is pushed into a cylinder that is connected to a piston. The gas in the cylinder then propels the piston, which then cycles the bolt much like the impingement system. Mikhail Kalashnikov’s AK-47 series of rifles also uses a gas piston system.