The semi-automatic version of the weapon can be registered as a short-barreled rifle in most states in the United States. The MPX submachine gun is only available for military and law enforcement use, while a semi-automatic only version is also available for the civilian market. The MPX is also available with different barrel lengths, ranging from 114 to 406mm (4.5- to 16-inch). It has a cyclic rate of fire of 850 rounds per minute (RPM). It comes with a collapsible stock and a free-floating rail. The standard variant of the MPX comes with an 8.0 inch (20.3 cm) barrel and is chambered in 9×19mm Parabellum. SIG MPX with ROC Marine Corps Special Service Company Operator SIG Sauer also used the same gas piston system to develop the SIG MCX carbine. However, since its inception, first party conversion kits have not been made available for either of the two production generations of the gun. The MPX, in its second generation, features a system that would allow for conversion from 9mm to. It features the SIG Sauer short stroke push-rod gas system to reduce the recoil and improve the reliability of the weapon. It was designed in 2013 and was released to the general public in 2015. These design features, rare in submachine guns, were chosen to enhance the safety of the user and to have a more reliable firearm.

It is a gas-operated firearm featuring a closed, rotating bolt. The SIG MPX is a gas-operated submachine gun designed and manufactured by SIG Sauer, and is primarily chambered in 9×19mm Parabellum. Short-stroke gas-operated, closed rotating boltġ0-, 20-, 30- or 35-round detachable box magazine

Sig mpx keymod accessories