Everything to know about the SIG MPX

The SIG MPX is a gas-operated submachine gun designed and manufactured in the United States by SIG Sauer.  For those who have owned one, it is referred to as the ultimate sub gun.  

We would expect nothing less in quality, reliability, and functionality from one of the premier defense manufacturers in the world. This workhorse model is primarily chambered in the 9×19mm Parabellum (9 mm Parabellum) cartridge. It provides a versatile platform with smooth action that features a short stroke gas piston system with a closed, rotating bolt to increase the accuracy of the weapon. 

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The design features of the MPX were chosen to enhance its safety and reliability of the firearm. The closed rotating bolt was designed to prevent water or dirt from entering the chamber and causing malfunctions as well as to add a suppressor.

Designed in 2013, the MPX was released to the public in 2015 and was a predecessor to the SIG MCX which appeared about the same time.  It features the SIG Sauer short stroke push-rod gas system to reduce recoil and improve the reliability of the weapon.

The second generation MPX features a system that allows the for conversion from 9mm to .357 SIG or .40 S&W variants.  The standard variant of the MPX comes with an 8.0 inch (20.3 cm) barrel and is chambered in the 9 mm Parabellum cartridge. It arrives from the factory with a collapsible stock and a free-floating rail and capable of a fire rate of 850 rounds/minute (RPM). The MPX require a proprietary 30-round translucent magazines that are easily stackable. 

The SIG MPX is also available with different barrel lengths, ranging from a stubby 4.5- to standard 16-inches.  A semi-automatic version is available for the civilian market, while MPX submachine gun is only available for military and law enforcement use.  Like the SIG MCX, the semi-automatic version of can be registered as a short barreled rifle (SBR) in most states in the United States. The barrels are often threaded (13.5 x 1 metric left-handed threads) for a muzzle attachment. 

The standard SIG MPX submachine gun comes with a full length picatinny rail system on the top of the weapon but no sights. The generation 2 MPX comes with a flat faced Timney trigger and M-LOK rail.  The SIG MPX have many of the same controls and manual of arms as the AR-15.

Sig Sauer MPX PCB 9mm 4.5” Pistol, Black - PMPX-4B-9 | Palmetto State Armory
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It features an ambidextrous selector switch and an ambidextrous bolt catch.  The exterior looks like this design is a cross between and AR-15 and H&K MP5.  The charging handle and last round bolt hold-open are mirrored from the AR-15. The magazine release, bolt catch, and safety are bilateral for easy access.

The MPX’s gas-operated piston-driven design produces an unnaturally smooth firing cycle as a result in these light pistol calibers. It sports a free-floating barrel surrounded by polymer/steel Picatinny rail mounts or M-LOK mounts.  The charging handle of the weapon was placed at the back of the weapon and largely based on the AR-15 design. This ensures that charging the weapon does not interfere with optics mounted onto the Picatinny rail system. The rails are made out of cast steel and the standard configuration weighs 4.6 lbs.

The SIG MPX is a standard pistol variant with a three-position collapsible arm support with a stock like appearance, a three-position fire selector if it supports full automatic, and an 14” blind pined and welded muzzle brash. Buttstocks are easily changed in the field by the shooter.  This weapon includes plenty of KeyMod space for accessories.  

The SIG MPX-K compact variant sports a 4.5 inch barrel.  The MPX K is the newest variant appearing in 2019 and also uses the closed bolt gas piston system used by military and law enforcement professionals all over the world because of its reputation reliability and robustness.  The MPX K has a modular system allowing for multiple accessories including a handguard and a choice of barrel lengths ready for suppressor use.

Later versions of the K features the SIG pistol pivoting contour folding brace (PCB) folding brace allowing the K to fold into an more compact size and still operate in the folded position. The K also features an updated M-Lok handguard and is available as accessories in multiple lengths for suppressor coverage. Finally, the K has been upgraded with the Timney trigger. Timney and SIG Sauer co-developed this trigger to maximize reliability for it to survive the roughness of a submachine gun platform. 

Other variants include the MPX-SD, the suppressed variant, with an MP5SD-like integral suppressor, a longer fore-end, and an 8.0 inch barrel.  The MPX Pistol is a semi-automatic only pistol variant with no stock and an 8.0 inch barrel. The MPX PSB is a semi-automatic only pistol variant with a SIG pistol stabilizing brace (SBX) and an 8.0 inch) barrel. 

Sig MPX Carbine 9mm Keymod MPX-C-9-KM-T Range Model | Palmetto State Armory
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The MPX-C is a semi-automatic only carbine variant, featuring a 6.5 inch barrel with a 9.5 inch muzzle device permanently welded to it.  The US Bureau of Alcohol Tobacco and Firearms (BATF) determined this would require additional permits and licenses to own.  After losing a civil case to try to resolve the matter, SIG USA soon replaced it with a carbine variant with a 16 inch barrel without the muzzle device and designated it as the MPX PC.

Every aspect of the SIG MPX platform has been rigorously designed and tested. Among the best features are the familiar AR controls.  This system performs well in adverse conditions. While the SIG MPX provides users the versatility and freedom it is also expensive compared to other platforms. The reputation of SIG Sauer and SIG USA is not surpassed and the exquisite design for reliability and functionality make the price a nice value.


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Written by Nicholas