SIG MPX

Design details

The standard MPX variant has an 8.0-inch barrel and a collapsible stock with a free-floating rail. Barrel lengths range from 114 to 406 mm. The submachine gun version is restricted to military and law enforcement, while a semi-automatic version is available for civilians.

The semi-automatic model can be registered as a short-barreled rifle in most US states. The standard MPX lacks sights and uses a full-length Picatinny rail; Gen 2 models have a flat-faced Timney trigger and M-LOK rail. Its short-stroke gas piston system operates from a closed bolt to keep water or dirt out of the chamber and to facilitate suppressor use.

An ambidextrous selector switch, bolt release, and magazine release are standard. The charging handle is at the rear, based on the AR-15 design, to avoid interfering with optics. The rails are cast steel, and the overall weight is 2.1 kg.

Variants

A pistol variant has a three-position collapsible arm support and barrel lengths of 14, 16, 8.0, 6.0, or 4.5 inches. A compact variant uses a 114 mm barrel. A fully suppressed variant includes an MP5SD-like integral suppressor, a longer fore-end, and a 203 mm barrel. Semi-automatic pistol variants exist with no stock and an 8.0-inch barrel, or with a SIG SBX stabilizing brace and an 8.0-inch barrel; in 2015 the BATFE warned that shouldering a braced weapon constituted making a Title II NFA weapon, but withdrew this opinion in early 2017.

Another semi-automatic pistol variant has a 114 mm barrel. A semi-automatic carbine variant, the MPX-C, has a 165 mm barrel with a permanently welded 241 mm muzzle device; SIG called it a muzzle brake, but the BATFE ruled it a suppressor in 2013, classifying the weapon as a Title II NFA item. An appeal was rejected in February 2014, and a SIG civil lawsuit alleging arbitrary and capricious action was upheld by Federal Judge Paul Barbadora in September 2015. Subsequently, SIG announced the MPX PCC, a carbine with a 406 mm barrel and no muzzle device.