The SIG MCX is a series of rifles designed and manufactured by SIG Sauer, featuring a short-stroke gas piston system, carried over from the SIG MPX submachine gun. The rifle is produced in both selective fire and semi-automatic configurations. It is available in rifle, short-barreled rifle, and pistol (generally considered as a compact carbine) configurations.
SIG MCX with a standard folding stock
Personal defense weapon
Designated marksman rifle
|Place of origin||United States|
|Unit cost||MCX VIRTUS Patrol: MSRP $2,233 USD|
MCX VIRTUS SBR: MSRP $2,233 USD
MCX VIRTUS Pistol: MSRP $2,235 USD 
|Mass||2.61 kg (5.8 lb) (229 mm barrel)|
2.72 kg (6.0 lb) (292 mm barrel)
4.03 kg (8.9 lb) (406 mm barrel) (MCX-MR) 
|Length||730 mm (29 in) stock folded (229 mm barrel)|
908 mm (35.7 in) stock unfolded (406 mm barrel)
|Barrel length||229 mm (9.0 in)|
292 mm (11.5 in)
406 mm (16.0 in)
7.62×51mm NATO (MCX-MR)
.300 AAC Blackout
|Action||Short-stroke gas-operated piston, rotating bolt|
|Rate of fire||900 RPM|
|Muzzle velocity||MCX Carbine 5.56x45mm NATO: 3,000 ft/s (914 m/s) |
|Effective firing range||MCX Carbine 5.56x45mm NATO: 1,650 ft (503 m) |
|Feed system||30-round detachable STANAG box magazine|
10 or 20-round box magazine (MCX-MR)
|Sights||Picatinny rail for mounting iron or optical sights|
The SIG MCX was first introduced at SHOT Show 2015.
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.
- MCX Carbine: Carbine, with 16-inch (410mm) 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-inch (230mm) 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-inch (230mm) or 11.5-inch (290mm) 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 it is really 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 inches (410 mm), 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 pounds (4.0 kg), 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 Patrol: Semi-automatic only  Carbine introduced in 2017. It features a 16-inch (410mm) barrel, a 1:7 inch twist, a custom Sig Matchlite Duo Trigger for improved accuracy, a folding and collapsing 5-position stock, four handguard lengths to choose from, interchangeable barrels, and a special internal recoil system.
- MCX VIRTUS SBR: Short-barreled rifle version of the VIRTUS Patrol, the 5.56x45mm NATO barrel is 11.5-inches (290mm) long, and the .300 AAC Blackout barrel is 9-inches (230mm) long.
- MCX VIRTUS Pistol: Semi-automatic pistol version of the VIRTUS Patrol, the 5.56x45mm NATO barrel is 11.5-inches (290mm) long, the .300 AAC Blackout barrel is 9-inches (230mm) long, and the VIRTUS Pistol features an SBX stabilizer brace in place of the stock.
- Australia: SIG MCX SBR variant used by Victoria Police Special Operations Group.
- Germany: 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, 531 units ordered 
- Israel: Sayeret Matkal use MCX Rattler in .300 AAC
- Indonesia: Indonesian National Police Special Response Unit: Gegana Brimob POLRI
- Malta: Armed Forces of Malta
- Netherlands: Netherlands Maritime Special Operations Forces
- Poland: Jednostka Wojskowa Grom use MCX and MCX Rattler in 300 BLK with conversion kits to 5.56×45mm NATO and 7.62×39mm ammunition,
- Portugal: Polícia de Segurança Pública SIG MCX 5,56mm used by Special Operations Group (Portugal).
- United Kingdom: Police Counter Terrorist Specialist Firearms Officer teams
- United States: USSOCOM Rattler conversion kit in .300 AAC Blackout
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