The SIG Sauer MPX is a gas-operated submachine gun primarily chambered in 9×19mm. It is the first submachine gun to feature a fully closed and rotating bolt and piston system.
The weapon, in its second generation, features a system that would allow for conversion from 9mm to .357 SIG or .40 S&W.
SIG Sauer used the same gas piston system to develop the SIG Sauer MCX carbine.