SIG MCX

The SIG MCX is a weapon series designed and manufactured by SIG Sauer, featuring a short-stroke gas piston system, carried over from the SIG MPX submachine gun. It is made in both semi-automatic and selective fire configurations. It is available in rifle, short-barreled rifle, and pistol configurations.

Built at SIG's New Hampshire factory, the MCX was "engineered from the ground up to be silenced, light and short" according to a SIG Sauer brochure.

In 2016, SIG recalled some weapons with the first generation bolt carrier group.

Design
The SIG MCX series features a short-stroke gas piston system to reduce recoil and improve the reliability of the weapon; this was based on the design of the earlier SIG MPX. The MCX features a system that allows for conversion between 5.56×45mm NATO, .300 AAC Blackout and 7.62×39mm ammunition, using standard 5.56 mm STANAG magazines for 5.56×45mm NATO and .300 AAC Blackout, and specially designed STANAG-compatible magazines for 7.62×39mm. The MCX is designed to deliver optimal performance with .300 AAC Blackout and an optional suppressor.

The barrel's profile is tapered at the crown to allow the installation of muzzle devices and direct-thread sound suppressors without the use of washers that degrade performance and allows the devices to self-center on installation. The barrel can be changed in a matter of seconds to another length or a different caliber. Additionally the barrels are nitride coated for corrosion resistance. It features hardened steel wear points.

All MCX variants have a forend made of aluminum with a KeyMod system to add accessories. Controls are mostly ambidextrous including the charging handle but not the bolt release. Four types of stocks are available for the MCX carbine.

SIG designed the upper receiver to be compatible with standard AR-15 and M16 lower receivers with the help of an adapter. The overall layout of the two rifles is similar.

Variants

 * MCX Carbine: Carbine, with 16 in barrel. Available with a safe/semi-automatic trigger group for U.S. civilian sales, or safe/semi-automatic/fully automatic trigger group for sales to law enforcement agencies and militaries.
 * MCX SBR: Short-barreled rifle, with 9 in barrel. Available with a safe/semi-automatic trigger group for U.S. civilian sales, or safe/semi-automatic/fully automatic trigger group for law enforcement and military sales. Under U.S. federal law, rifles with barrels shorter than 16 inches are Title II weapons, which are subject to federal restrictions, as well as being regulated by state laws.
 * MCX Pistol w/ SBX: Semi-automatic pistol, with 9 in barrel, SIG SBX stabilizing brace and safe/semi-automatic trigger group. This version fits the U.S. legal definition of a "handgun", in that it is only designed to be fired with a single point of contact with the shooter's body, though in general terms, it is a compact carbine rifle, as it fires an intermediate round. The BATFE previously warned users that shouldering a weapon fitted with the SIG SBX, or a similar forearm brace, and not registered as a short-barreled rifle, constitutes the making of a short-barreled rifle, which is a Title II weapon. However, as of April 2017, this is no longer the case.
 * MCX Low Visibility Assault Weapon (LVAW): Short-barreled, suppressed, select-fire variant available only to law enforcement agencies and militaries. It is nicknamed "Black Mamba".
 * MCX-MR (Mid Range): Semi-automatic sniper rifle chambered in 7.62×51mm NATO. It features a 16 in, fluted, 416 stainless steel barrel, with a 1:10 inch twist, manufactured by Bartlein Barrels. The handguard is secured via two screws, and the gas system has settings for suppressed or unsuppressed use. It weighs 8.9 lb, and features both an M16/AR-15-type charging handle and a left side charging handle, uses SR-25 box magazines, and is compatible with SR-25 lower receivers. It was SIG Sauer's unsuccessful submission for the United States Army's Compact Semi-Automatic Sniper System (CSASS) program.
 * MCX Virtus: In 2017 Sig Sauer announced an update to their MCX line of rifles renaming the series MCX Virtus. The MCX Virtus rifles have redesigned upper receivers, handguards, barrels, and bolts which are incompatible with prior MCX versions. In addition all mention of 7.62x39mm barrel kit references have been removed for the sight; limiting options to 5.56x45mm and .300 Blackout.

Users

 * 🇳🇱: Netherlands Maritime Special Operations Forces
 * : Police Counter Terrorist Specialist Firearms Officer teams
 * : Indonesian National Police Special Response Unit: Gegana Brimob POLRI
 * Berlin Police, 300 modified rifles to meet the police's requirements were ordered in October 2017 plus an additional 160 rifles for the SEK of Berlin
 * Landespolizei of Schleswig Holstein, 522 units ordered
 * Landespolizei of Schleswig Holstein, 522 units ordered

Orlando nightclub shooting
A SIG MCX was used in the 2016 Orlando nightclub shooting, which was the second deadliest mass shooting in U.S. history.