SIG MCX: A High-Performance Firearm for Military and Civilian Use

Eric Sof

SIG Sauer MCX Virtus Patrol

The SIG MCX is a popular family of firearms manufactured by SIG Sauer, available in the selective fire and semi-automatic models. The MCX features a short-stroke gas piston system derived from the SIG MPX submachine gun. The rifle is designed to deliver optimal performance, reliability, and versatility. It is available in rifle, carbine, short-barreled rifle, and pistol configurations, making it an excellent option for various applications. This article will explore the features, history, and applications of the SIG MCX.

A Brief History

The SIG MCX was first introduced at the SHOT Show in 2015. SIG USA, a subsidiary of SIG Sauer based in New Hampshire, designed the rifle. The rifle features a short-stroke gas piston system based on the design of the earlier SIG MPX submachine gun. The MCX was originally designed to deliver optimal performance with .300 AAC Blackout ammunition and an optional suppressor. However, the rifle’s versatility and adaptability have made it popular for various applications.

SIG Sauer MCX rifle
SIG Sauer MCX rifle (Photo: XY)

In 2016, SIG recalled some of the rifles that had the first-generation bolt carrier group. This issue was quickly resolved, and the SIG MCX has since become a popular choice among military, law enforcement, and civilian shooters. Unfortunately, a SIG MCX was used in the 2016 Orlando nightclub shooting, which at the time was the deadliest mass shooting in U.S. history, now second to the 2017 Las Vegas shooting. This event highlighted the need for responsible gun ownership and safe storage of firearms.


The SIG MCX is designed to deliver optimal performance, reliability, and versatility. The rifle features a short-stroke gas piston system, which reduces recoil and improves the weapon’s reliability. The MCX can be converted 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. This makes the MCX a versatile rifle that can be adapted to various applications.

The MCX features a tapered barrel crown, which allows the installation of muzzle devices and direct-thread sound suppressors without using washers that degrade performance. The barrel can be changed in a matter of seconds to another length or a different caliber, which makes the MCX a highly adaptable rifle. The barrels are nitride coated for corrosion resistance and feature hardened steel wear points, ensuring the rifle’s durability.

SIG MCX the right side view
SIG MCX the right side view (Photo: XY)

The first generation MCX variants have a forend made of aluminum with a KeyMod system to add accessories, while second generation MCX variants have an M-LOK handguard. Controls are mostly ambidextrous, including the charging handle but not the bolt release. Four stock configurations are available for the MCX carbine, making it a customizable rifle that can be tailored to individual needs. The overall layout of the two rifles is similar, and the upper receiver is designed to be compatible with standard AR-15 and M16 lower receivers with the help of an adapter.

In July 2018, USSOCOM, the United States Special Operations Command, selected an integrally suppressed upper receiver group for the SURG (Suppressed Upper Receiver Group) contract based on the MCX platform. The suppressed uppers were intended to be paired with the M4A1 lowers already in the SOCOM inventory. This decision was made to enhance the capabilities of the existing weapons and provide military personnel with an effective means of suppressing the noise produced by firearms during combat operations. By opting for the integrally suppressed upper receiver group, USSOCOM intended to improve the effectiveness and versatility of their firearms while ensuring compatibility with the existing inventory.

The Short-Stroke Gas Piston System

One of the most significant features of the SIG MCX is the short-stroke gas piston system. The system is designed to reduce recoil and improve the weapon’s reliability, making it more controllable and comfortable to shoot. The system is inherited from the earlier SIG MPX submachine gun and has been adapted to fit the MCX.

SIG MCX (Photo: XY)

The short-stroke gas piston system works by diverting gas from the barrel to operate the action instead of using the gas to drive the bolt carrier group directly, as in a direct impingement system. This design reduces the amount of fouling in the receiver and bolt carrier group, improving the weapon’s reliability and making it easier to maintain.

The MCX can also fire various types of ammunition, thanks to the conversion system that allows the shooter to change the caliber or length of the barrel in just seconds. This makes the MCX a versatile weapon, capable of adapting to a wide range of shooting situations.


The SIG MCX is a versatile and widely-used rifle in various configurations. The rifle is available with a safe/semi-automatic trigger group for the U.S. civilian market. In contrast, military and law enforcement agencies can obtain it with a safe/semi-automatic/fully automatic trigger group.

SIG Sauer offers three different semi-automatic rifle configurations for the civilian market. The SIG MCX Patrol is the standard configuration with a 406 mm (16 in) barrel. The SIG MCX SBR is a short-barreled rifle configuration with a 229 mm (9 in) barrel subject to federal restrictions and regulated by state laws. The SIG MCX Pistol is the pistol configuration of the rifle that fits the U.S. legal definition of a handgun. It has a 229 mm (9 in) or 292 mm (11.5 in) barrel and comes with either the SIG Sauer SBX or SIG Sauer PCB. The BATFE previously warned users that shouldering a weapon fitted with the SIG SBX or similar forearm brace and not registered as a short-barreled rifle constitutes the making of a short-barreled rifle is a Title II weapon. However, this is no longer the case as of April 2017.

SIG MCX the left side view
SIG MCX the left side view (Photo: XY)

The SIG MCX Low Visibility Assault Weapon (LVAW) is a short-barreled, suppressed, select-fire variant available only to military and law enforcement agencies. It is nicknamed the “Black Mamba.”

The second generation of the SIG MCX series is the SIG MCX VIRTUS, introduced in 2017. The SIG MCX VIRTUS Patrol is the standard configuration rifle with a 406 mm (16 in) barrel, custom Sig Matchlite Duo Trigger for improved accuracy, and folding and collapsing 5-position stock. It has four handguard lengths, interchangeable barrels, and a special internal recoil system. The SIG MCX VIRTUS SBR is the short-barreled rifle configuration of the MCX VIRTUS with a 292 mm (11.5 in) barrel for the 5.56×45mm NATO caliber and a 140 mm (5.5 in) or 229 mm (9 in) barrel for the .300 AAC Blackout caliber. The SIG MCX VIRTUS Pistol is the pistol configuration of the MCX VIRTUS with an SBX stabilizer brace and a 292 mm (11.5 in) barrel for the 5.56×45mm NATO caliber and a 229 mm (9 in) barrel for the .300 AAC Blackout caliber.

The SIG MCX RATTLER is a short-barreled rifle variant designed for personal defense and chambered in .300 AAC Blackout and 5.56. It features a 140 mm (5.5 in) barrel and a Picatinny rail tail interface for attaching a compact buttstock or a folding PCB. In February 2018, USSOCOM ordered upper receiver group conversion kits for the MCX Rattler in .300BLK for evaluation. The MCX Rattler was later selected as the winner of SOCOM’s Commercial Personal Defense Weapon (CPDW) contract in May 2022, and suppressed MCX Rattlers will be ordered in 5.56 and .300BLK.

The MCX Rattler has been reported to be a more dependable firearm than the M4/M4A1. The M4 uses a direct impingement system, while the Rattler features a combined gas piston system and dual recoil springs that effectively minimize jamming issues. This is because the Rattler’s gas piston system is more efficient in managing the recoil and preventing malfunctions. This mechanism helps to ensure that the Rattler is a more reliable and efficient firearm than the M4/M4A1.

The third generation of the SIG MCX is the SIG MCX-SPEAR LT, introduced in 2022. It is designed as a carbine with a 9-inch (230 mm), 11.5-inch (290 mm), or 16-inch (410 mm) barrel and a Picatinny rail tail interface for attaching either a buttstock or a pistol brace.


The SIG MCX Spear is a further development of the MCX and was adopted by the U.S. Army in 2022. The MCX Spear is chambered in the .277 Fury cartridge, a new round developed by SIG Sauer specifically for the Army’s Next Generation Squad Weapon (NGSW) program.

SIG Sauer NSGW (Photo: XY)

The .277 Fury cartridge is designed to deliver superior accuracy, lethality, and range compared to the standard 5.56×45mm NATO cartridge. The round has a maximum effective range of 1,000 meters and can penetrate Level IV body armor at 600 meters. The MCX Spear is designed to exploit these capabilities, making it a potent weapon for modern warfare.

The SIG MCX in the News

The SIG MCX has been in the news for several reasons, some controversial. In 2016, a SIG MCX was used in the Orlando nightclub shooting, which was, at the time, the deadliest mass shooting in U.S. history, though the 2017 Las Vegas shooting has since surpassed it. The shooter used a modified SIG MCX, which had been converted to full auto and had an illegally obtained suppressor attached.

After the shooting, SIG Sauer recalled some first-generation MCX rifles with the bolt carrier group. The recall affected several rifles, and SIG Sauer worked closely with the ATF to resolve the issue. The company also introduced several design changes to address the concerns raised by the recall.

Despite this incident, the SIG MCX remains popular among shooters and law enforcement agencies due to its reliability, versatility, and accuracy.

Technical specifications

Country of origin:Germany / Switzerland / USA
Manufacturer:SIG Sauer
Entered service:2015
Caliber:5.56×45mm NATO; .300 AAC Blackout
Weight:2.61 kg (5.8 lb) (229 mm barrel); 2.72 kg (6.0 lb) (292 mm barrel); 3.6 kg (7.9 lb) (406 mm barrel)
Length (stock unfolded):730 mm (29 in) for 229 mm barrel
Length (stock unfolded):908 mm (35.7 in) for 406 mm barrel
Barrel length:229 mm (9.0 in); 292 mm (11.5 in); 406 mm (16.0 in)
Muzzle velocity:MCX Carbine 5.56×45mm NATO: 3,000 ft/s (914 m/s)
Cyclic rate of fire:800 rounds per minute
Practical rate of fire:30 – 90 rpm
Magazine capacity:908 mm (35.7 in) for a 406 mm barrel
Sights:Picatinny rail for mounting iron or optical sights
Range of effective fire:MCX Carbine 5.56×45mm NATO: 1,650 ft (503 m)

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