The SIG MPX is a gas-operated submachine gun designed and manufactured by SIG Sauer, and is primarily chambered in 9×19mm Parabellum cartridge. It is the first submachine gun to feature a fully closed and rotating bolt and piston system.This was done to enhance the safety of the user and to have a more reliable firearm. It was designed in 2013 and was released to the general public in 2015. It features the SIG Sauer short stroke push-rod gas system to reduce the recoil and improve the reliability of the weapon.
The MPX, in its second generation, features a system that would allow for conversion from 9mm to .357 SIG or .40 S&W, however, since its inception, first party conversion kits have not been made available for either of the two production generations of the submachine gun.
SIG Sauer also used the same gas piston system to develop the SIG Sauer MCX carbine.