Sig Sauer introduced its MPX submachine gun in 2013 with hopes of capturing some of the law enforcement market that had been long held by the Heckler & Koch MP5. The select-fire gun uses a short-stroke gas piston system and a rotating bolt for increased accuracy and reduced felt recoil. It also possesses completely ambidextrous controls. In 2015, several semi-auto variations were made available to the civilian market with one of the more interesting, at least to me, being the MPX-K PSB.

Classified by the BATFE as a handgun, the MPX-K PSB has a 4.5-inch barrel, a free-floating KeyMod handguard and a collapsible arm brace. It is a compact personal-defense weapon (PDW) capable of being concealed in just about any briefcase or messenger bag yet easily deployed to provide fast and accurate hits.

Despite its small footprint, the MPX-K PSB is not a lightweight by any means. Though the upper and lower receivers are made from aluminum, the gun still weighs in at 6.2 pounds yet balances nicely between the hands. With a loaded 30-round magazine locked in place, the gun’s weight is a solid 7 pounds. Its overall length with the stock collapsed is just 17.5 inches and only 22.5 inches with it fully extended.

Hands On

The MPX family of guns all fire from a closed bolt. Whereas most of the 9mm ARs on the market are blowback in operation, Sig engineers designed the MPX with a short-stroke gas piston to impart enough energy on the gun’s carriage system to unlock the bolt, extract the fired casing and eject it. As the bolt moves rearward, it compresses dual recoil springs that then push the bolt forward, stripping a new cartridge from the magazine and chambering it. The piston system theoretically makes the gun more reliable with a broader selection of ammunition compared to blowback-style guns, and it’s also a better choice for work with a suppressor.

Without the AR-style buffer tube, the MPX-K PSB is a very compact weapon. The telescoping arm brace can be expanded by simply pushing a spring-loaded button and pulling it rearward. It has three positions: all the way closed, midway between closed and fully opened, and all the way open. Locked in its fully open position, the brace provides the user with a length of pull just over 12 inches, and I found it very comfortable to use while shooting.

ATF Compliant

Sig introduced the SB15 Pistol Stabilizing Brace back in 2013, and offered what had previously been classified as short-barreled rifles (SBRs) with these rubber braces. Designed to help wounded warriors and other shooters with physical limitations fire the pistols with one hand, the BATFE initially issued a letter saying that it was okay to place the arm brace against the shoulder when firing. Shooters immediately discovered that this legal loophole allowed them to enjoy all the benefits of a SBR without doing reams of paperwork and paying $200 for a tax stamp.

The popularity of stabilizing braces skyrocketed until the BATFE reversed itself and said that placing such a brace against one’s shoulder constitutes a redesign of the weapon, in this case a handgun, making it an illegal SBR. The BATFE did not prohibit the ownership or attachment of a brace—just using it like a shoulder stock. But, in a surprise move, the BATFE reversed itself yet again in April of 2017 and said that shouldering a handgun outfitted with an arm brace did not constitute a redesign of the pistol and can be legally shouldered and fired. Sig includes a copy of the BATFE letter with each MPX-K PSB.

MPX-K PSB Controls & Features

The safety is located in the AR-15/M16 position, and the gun has the same grip angle, making its use very familiar to anyone who has trained with the 5.56 platform. Additionally, the safety, magazine release and bolt release are all ambidextrous and can be operated from either side of the weapon. Also, I found the right-side bolt release to be perfectly positioned for activation with my right-hand trigger finger, and it didn’t require much pressure to drop the bolt. Sig also ships the MPX-K PSB with one 30-round, translucent polymer magazine with reinforced feed lips.

For targeting, Sig sent me a ROMEO3 miniature reflex sight. The provided graphite high mount positioned the sight perfectly on the gun’s top rail. I like it a lot! It’s fast and easy to pick up, and the dot’s intensity can also be adjusted for any lighting situation. Its relatively small 3-MOA dot allows for precision yet is still extremely fast for the eye to acquire. One interesting feature of the ROMEO3 is that is has a motion-activated illumination system that powers up when it senses motion and powers down when the gun isn’t being handled. The housing utilizes machined aluminum, and it is also waterproof. It uses a QD mount that made it easy for me to find the perfect position for it on the MPX-K PSB’s top rail, and the whole unit weighs just 4.4 ounces.

Range Time

I started by testing the MPX-K PSB by shooting groups at 25 yards, but they were so unbelievably small that I moved my target out to 50 yards. For these longer shots I mounted a Trijicon 1-4x24mm AccuPoint scope and dialed up the magnification so I could get an indication of the pistol’s true mechanical accuracy.

I fired all of the groups from my DOA Tactical portable shooting bench and used a rifle rest for support. Unfortunately, the length of the 30-round magazine made it impossible to use the rest with it locked in place. So, I loaded and chambered each round from the magazine, removed it and fired a single shot. The groups ran from just over 1 inch to 1.5 inches, with the average of all loads being just 1.25 inches! Remember, this is a handgun with a 4.5-inch barrel, so this kind of accuracy is pretty incredible.

The trigger pull on my test gun was a crisp 5.5 pounds. It was consistent and predictable, and I had no problem at all shooting tight groups. It also had a very firm reset, and that was a big help in shooting the gun rapidly.

I set up my MGM BC-C Zone steel target at 15 yards and used a PACT electronic timer to measure my splits, or time between shots, firing double-taps. Using Sig’s 124-grain FMJ rounds, I found that I could place both hits on target in less than 0.16 seconds. Shooting with both eyes open and the ROMEO3 dot set so I could easily find it even in the bright sunlight, it was an easy matter to double-tap the steel at this distance, without failure, even when pushing for speed. The Sig is so controllable that the little 3-MOA dot never moved off target.

Flawless Fighter

I fired over 300 rounds during my evaluation and didn’t have a single failure of any kind. I used +P ammo and bullet weights from 77 grains up to 150 grains, and the MPX-K PSB chewed them up and spit them out. Also, even the cheap 115- grain FMJ range fodder ran fine in the handgun!

The Sig MPX-K PSB is an impressive pistol! Its short-stroke gas system and rotating bolt make it controllable, light on recoil and extremely accurate. Its ability to place a large number of rounds on target in a rapid manner is impressive. Moreover, in the right hands, this is a devastating weapon, and its small size makes the MPX-K PSB an easily concealable PDW. The MPX line continues to expand, with the likes of ultra-compact SIG Copperhead leading the way. There simply is no slowing down the MPX series.

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SIG Sauer MPX-K PSB Specifications

  • Caliber: 9mm
  • Overall Barrel Length: 4.5 inches
  • Overall Length: 17.5-22.5 inches
  • Grip: Polymer
  • Overall Weight: 6.2 pounds (unloaded)
  • Sights: None
  • Action: Piston-operated semi-auto
  • Finish: Matte black
  • Capacity: 30+1
  • MSRP: $2,162

This article is from the 2019 issue of Black Guns magazine. Grab your copy at

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