Sig Sauer® MPX SBR

By Thomas Murphy

The SIG MPX Short Barrel Rifle (SBR) is based on the SIG MPX gas-operated submachine gun manufactured by SIG SAUER. In the 1970s, Schweizerische Industrie Gesellschraft (SIG) purchased both Hammerli and J.P. Sauer and Sonn and was reborn as SIG SAUER. There are now two SIG SAUER companies, one in New Hampshire, USA, and the other in Ekenforde, Germany.

In January 1985, SIGARMS went into business in Tyson’s Corner, Virginia where the P220 and P230 handguns were imported from Germany. In 2007, SIGARMS underwent a name change to SIG SAUER Inc. The company’s fortunes had been on a downhill run since 2004, with less than 140 employees. CEO Roy Cohen added firearms based on the AR-15 platform, and the company grew exponentially up to 2016 when there were over 1,000 employees producing over 43,000 firearms per year. Current production is based in Exeter, NH.

In 2013, the MPX submachine gun was introduced. It fires from a fully closed rotating bolt and piston assembly. This was done primarily for safety and reliability reasons. Rate of fire is right at 850 rounds per minute, which is rapid, but fully controllable due to the low recoil of the 9mm pistol round. Second round target acquisition is quite fast.

The second generation MPX was redesigned to be converted to shoot 9mm, .357 SIG or .40S&W. SIG calls the system Multi-Cal, however, right now, the only chambering available is the 9mm. The MPX can be fitted with carbon steel barrels in lengths from 4.5 to 16 inches. The MPX is only available to military or law enforcement personnel.

The gas operating system is designed to prevent liquid or dirt from entering the chamber which could cause a malfunction. The closed bolt also makes for better operation with a suppressor. Instead of the bolt slamming back as the gun is fired, there is just enough delay (slight) to allow gas pressure to drop, therefore reducing the sound signature. When fired with subsonic ammunition, the MPX sounds like a standard velocity .22 Long Rifle.

In a side view, the MPX resembles a cross between an H&K MP5A3 and an M-4 carbine. It has an ambidextrous safety, bolt catch, magazine release and selector switch. The M-4-type cocking lever allows any type of optics from red dot, to night vision to be mounted on the cast steel Picatinny top rail.

The gun tested is a semi-automatic version of the MPX–the MPX SBR. It mounts a 4.5-inch barrel, and this unit has a SIG SRD9 suppressor installed. Transfer of this MPX will require two tax stamps. Its gas system will handle all brands of 9mm ammunition and bullet weights from 115 grains to 147 grains without adjustment. Specialty ammunition like the NovX Engagement 65-grain ammo might require some adjustment. This author fired about 200 rounds of Super Vel “Hush Puppy” 147-grain FMJ subsonic rounds with nary a problem. Then the MPX was fed a fairly-hot diet of Super Vel +P 90 grain JHP 1513 fps rounds with the suppressor installed. Not anywhere near as quiet as the Hush Puppy which slides along at 950 fps but still reduced the report and fed with zero problems. The MPX comes with one 30-round magazine, and both a 10-round and 2-round are available as options. Both magazines are the same length with the 10-round having a block off plate to restrict capacity.

Iron sights come mounted on the Picatinny rail. Both front and rear sights are adjustable and removable. These sights are designed to be a simple durable sighting solution should the optical device fail. The rail system is marked to provide a reference point to reattach the sights without losing zero. Both the front and rear sights can be removed by pressing a release button on their bases.

The KeyMod handguard has attachment points for installing 2-inch and 4-inch accessory rails, which can accept lights, LASERs or other accessories. The handguard can be removed for barrel access by sliding the front pivot pin about half way open and then removing the guard.

Removal of two T-25 Torx barrel retention screws from the right side of the upper receiver allows the barrel to be removed. The 4.5-inch barrel can be replaced by barrels up to 16 inches in length. The MPX came with a flash suppressor, which had to be removed in order to fit the SRD9 suppressor. The removable handguard allowed a wrench to fit the flash suppressor’s hex nut and the SRD9 was threaded on to the barrel.

Disassembly is similar to an AR-type platform. Remove the magazine. Check that the chamber is empty. Insure that the bolt is fully forward, then push the rear takedown pin to the left and tilt the upper receiver off the lower. The bolt carrier can then be removed using the same method as on an AR. The charging handle can then be removed. This will be sufficient disassembly for most cleaning sessions.

The MPX was tested with five different types of 9mm ammunition; Super Vel Hush Puppy 147 grain subsonic, Super Vel +P 90 grain JHP, Black Hills 115 grain JHP, Black Hills 124 grain JHP and Ruger ARX+P 90 grain JHP. The Ruger ARX uses an injection-molded copper/polymer bullet developed by Polycase Ammunition that will penetrate numerous barriers without expansion. The non-expanding bullet distributes the bullet’s energy in a lateral direction via flutes in the bullet ogive. This bullet will penetrate clothing without deforming or clogging.

All testing was done at an indoor range under controlled temperature and conditions. There was no failure to fire, no stove-piping and no malfunctions of any sort. The suppressor was fitted for all shooting. As expected, the subsonic Super Vel 147 grain Hush Puppy was the quietist of all five rounds. Accuracy at 15 yards stayed in the 1.5-2.3-inch range.

During testing, it was noticed that the trigger had a slight snap as the rotating bolt rode up on it as it reset. This was not objectionable, just something slightly different. Trigger let off was about average for a military-style SBR developed from a submachine gun. The gun was a real pleasure to shoot, as its 6 lb weight (with suppressor) soaked up almost all the recoil. All in all over 300 rounds were fired, and two of the shooters really wanted to take the MPX home on a permanent basis.


Two different optics were tested; the Aimpoint Patrol Rifle Optic red dot and the Trijicon Reflex with tritium insert.

Aimpoint Patrol Rifle Optic

This optic was designed with input from law enforcement and other professionals. It uses a hard anodized 30mm aluminum tube powered by a 3 volt lithium battery, type 2L76. Its projected battery life in hours exceeds 30,000. That’s over 3 years of continuous use. It is night vision compatible, partially due to its anti-reflex surface coating and the multi-layer coated objective lens. It has no magnification but can be used in conjunction with Aimpoint’s 3X magnifier. It’s compatible with all generations of night vision equipment and is sealed against dust and waterproof to 150 feet. The on-off switch also controls the red dot size so that the sight can be used during daylight conditions, at twilight and in full night scenarios.

Its modular mount is set up to fit directly on the SIG MPX without adaptors. Front and rear lens covers aid in keeping dust and grit off the lenses. The rear cover is transparent, and the sight can be used with the cover in place. Weight with the integrated mount is11.6 oz. From personal use, I’d have to say this is one of the most indestructible red dot sights ever seen.

Trijicon Reflex

The Reflex is a 4.5 MOA amber dot sight with a flattop mount specifically tailored to fit rifle platforms like the AR-15, M4A1 and SIG MPX family. It was developed for the US Special Forces. From playing with it for a few months, I’d have to say it’s about as fragile as an anvil. It’s typical Trijicon ACOG technology; the tritium insert insures no possible chance of battery failure; a bright light in low light, or zero light operations; extreme ruggedness and is designed for both eyes open shooting. Using both eyes allows quick target acquisition and improved situational awareness.

The reflex comes with a flattop rail adaptor, dust cover and polarizing filter. It’s 4.7 inches long and weighs 4.7 ounces without the RX14 mount. The tritium isotope uses fiber optics to light the reticule. The housing material is cast A357 aluminum.

This article first appeared in Small Arms Review V22N4 (April 2018)
and was posted online on February 23, 2018


Comments have not been generated for this article.