The SIG MPX is a submachine gun designed to operate with a fully closed and locked rotating bolt system. It employs a gas-operated short stroke piston system and the bolt is held open on the last round fired. The MPX can be configured for three calibers, 9mm, .357SIG, and .40S&W, by swapping out for the appropriate barrel. An integral picatinny rail at 12 o’clock allows for the use of optics, including the SIG MPX reflex sight. The ambidextrous safety/selector switch and magazine release are quite reminiscent of the AR-style. The stock system is user-configurable, with options for a telescoping, pistol, or folding stock. Magazines comes in 10/20 or 30 round configurations. The MPX itself comes in five styles: MPX (standard), MPX-K (compact variant), MPX-P (pistol variant), MPX-SD (integrally suppressed), and MPX-C (carbine variant).