SIG MPX

The SIG MPX is a submachine gun designed by SIG Sauer in 2013 and released in 2015, chambered primarily in 9x19mm Parabellum. Its defining characteristic is that it is gas operated and fires from a closed rotating bolt - both rare in submachine guns, which are usually blowback weapons firing from an open bolt. SIG chose the arrangement for safety and reliability: a closed bolt keeps water and dirt out of the chamber and makes suppressor use easier, while the short-stroke push-rod gas system reduces recoil. The same gas system was subsequently used to develop the SIG MCX carbine.

The standard weapon has an 8-inch barrel, a collapsible stock, a free-floating handguard and a cyclic rate of 850 rounds per minute, weighing 2.7 kg. Barrels from 4.5 to 16 inches are available. It ships without sights but with a full-length Picatinny rail on top; second-generation guns have a flat-faced Timney trigger and M-LOK handguard in place of the earlier polymer and steel Picatinny mounts. The selector, bolt release and magazine release are all ambidextrous, and the charging handle sits at the rear in the AR-15 position so that cocking does not interfere with mounted optics. The select-fire weapon is sold only to military and law enforcement customers; a semi-automatic version is offered commercially and can be registered as a short-barrelled rifle in most American states. The second generation was designed to allow conversion between 9mm, .357 SIG and .40 S&W, though SIG has never released first-party conversion kits for either production generation.

Selective-fire configurations include the standard MPX, the compact MPX-K with a 114 mm barrel, and the MPX-SD with an integral suppressor in the manner of the MP5SD, a longer fore-end and a 203 mm barrel. Semi-automatic models include the MPX Pistol, the MPX PSB with a stabilising brace, a semi-automatic MPX-K and the MPX-C carbine.

The MPX-C generated a notable regulatory dispute in the United States. Its 165 mm barrel carries a permanently welded 241 mm muzzle device that is in fact a modified version of the baffle core from the integrally suppressed variant. SIG maintained it was a muzzle brake, but the BATFE ruled in 2013 that the component was a suppressor and classified the weapon as an NFA Title II firearm. After the agency rejected SIG's appeal in February 2014, the company sued, alleging the BATFE had acted in an arbitrary and capricious manner; Federal Judge Paul Barbadora upheld the ruling in September 2015. SIG's response was the MPX PCC, a carbine with a 406 mm barrel and no muzzle device.