MPX-P: Sig Sauer's Semi-Auto Pistol

Sig Sauer MPX-P Semi-Auto Pistol lead — In testing, the Sig MPX-P was easy to handle and fast on target, offering light recoil and tight groups at 25 yards.

Sig Sauer MPX-P Semi-Auto Pistol solo — The incredibly versatile, piston-driven MPX-P pistol from Sig Sauer features a modular design that allows users to change barrels and handguards, use sound suppressors and even install stocks/braces to accomplish any mission.

Sig Sauer MPX-P Semi-Auto Pistol SBX — The small rail at the back of the receiver makes it easy to add Sig Sauer’s SBX stabilizing brace. Also note the sling attachment point at the heel of the polymer pistol grip.

Sig Sauer MPX-P Semi-Auto Pistol rear rail — You can use the rear rail section to install a sling loop, a QD sling attachment point or Sig’s SBX brace.

Sig Sauer MPX-P Semi-Auto Pistol handguard — The author’s test MPX-P came with an aluminum, rail-adaptable handguard, though KeyMod and carbon-fiber versions are available.

Sig Sauer MPX-P Semi-Auto Pistol flash suppressor — The A1-style flash suppressor is a big help, especially when it comes to preserving night vision in low light.

Sig Sauer MPX-P Semi-Auto Pistol magazine — Lancer Systems builds Sig’s MPX series magazines, which are made of translucent polymer with steel feed lips.

Sig Sauer MPX-P Semi-Auto Pistol controls — All of the MPX-P’s controls are ambidextrous and essentially identical to an AR’s for easier transition training.

The new Sig Sauer MPX-P pistol is a long way from where this 150-year-old Swiss wagon manufacturer began in the world of firearms—making muskets for its nation’s military. It does, however, make its own significant contribution to the company’s commitment to reliability, effectiveness and attention to detail.

RELATED STORY: SIGnified Sport Shooting: The 12 Pistols of the Sig Sauer XSERIES

While “Swiss made” is synonymous with quality, the MPX-P proves that quality can be replicated at Sig Sauer’s U.S. headquarters in Newington, New Hampshire, where the pistol is made. With one look, it is clear that the MPX was designed from the ground up as a submachine gun specifically for military and law enforcement use, and for these customers it is available in select-fire configuration.

But the rest of us aren’t left out in the cold, as several civilian-legal MPX variants are available, including the MPX-P pistol I recently got my hands on for testing. It is hardly surprising that Sig Sauer should be making a play for this market, as the company’s pistols are preferred by many federal law enforcement agencies, and Sig also offers high-quality rifles and carbines.

Xtreme Pistol

The MPX-P is a semi-automatic, 9mm pistol that uses a short-stroke piston to drive a rotating bolt. It was intentionally designed to mimic an AR in its controls and operation, but it is so much more. The main advantage in having AR-style controls is that most police officers who have trained with patrol rifles (and most everyone who has served in the military in at least the past 50 years) will be familiar with the AR. This vastly expedites and eases training and reduces user error in high-stress situations.

The upper and lower receivers are both made from extruded aluminum so that, despite its size, it is surprisingly lightweight. The MPX platform is also incredibly modular. With nothing but an Torx wrench, the 8-inch aluminum handguard on my test MPX-P can be removed and replaced with other options. It should be noted that the upper receiver is monolithic and that the interchangeable portion of the handguard only consists of the bottom portion under the barrel and a short portion above and to the front. This system also allows the barrel to remain free-floating for improved accuracy.

That same Torx wrench can also be used to easily and quickly remove the barrel/gas piston assembly, giving the user a choice of installing a 4.5-, 6.5- or 8-inch (included here) barrel. With the 8-inch barrel and handguard, only the A1 style flash suppressor is exposed, and there is a full 14 inches of uninterrupted Picatinny rail space for optics. The threaded barrel is also suppressor ready. My test model came with a handguard drilled and tapped for adding side and bottom rails, but carbon-fiber and KeyMod versions are also available.

The horizontal indentations at the rear of the upper receiver are designed to accept a fully collapsible stock. Fixed or folding stocks are also available for anyone who wants to register the pistol as a short-barreled rifle, or SBR. Another option is installing a Sig Sauer SBX stabilizing brace to offer greater stability while shooting. I have tested this system previously with a folding stabilizing brace, and it significantly increases both shooter comfort and ease of aiming.

The AR-style controls are fully ambidextrous, including the safety selector, magazine release and bolt release. The bolt catch remains in the traditional place on the left side of the receiver. The MPX-P also features an AR-style charging handle, but it is not compatible with any standard AR charging handle—it only looks similar and functions the same way. It does feature an enlarged latch for ease of use with one hand. There is also a fully functional polymer dust cover. On top of the receiver, Sig includes AR-style folding sights that, on my test gun, came zeroed from the factory and were very effective.

The trigger is an AR-type, single-stage unit, but my test gun’s seemed a bit on the heavy side at about 7.5 pounds. But then again, this was not designed as a target trigger but rather a submachine gun trigger. The integral triggerguard has also been enlarged to accommodate shooting while wearing gloves.

The magazine well is enlarged and aggressively beveled to make reloading faster and easier without having to look at the pistol. The curved magazines, which are available in 10-, 20- and 30-round capacities, are another incredibly well thought out feature. Instead of using plain metal magazines or going with a cheap alternative, Sig went with the best. Lancer makes the magazines for the MPX series; they feature translucent grey polymer bodies with unique, all-around steel inserts at the top that also form the feed lips. This makes for an extremely durable and reliable magazine, as anyone who has used Lancer’s AR magazines can attest. The action also locks back after firing the last round from the magazine.

The Sig-designed polymer pistol grip has an aggressive palm swell and beavertail as well as very aggressive texturing (almost like skater’s tape) on the sides. It also features a sealed internal storage compartment for extra batteries or small parts. At the rear of the lower receiver is a short Picatinny rail. This is primarily intended to serve as the anchor point for the various stock options. There is also a quick-detach (QD) sling swivel attachment point directly below the rail, and a single-point sling with a QD mount was included with my test gun.

The MPX-P disassembles in a manner similar to an AR with front and rear takedown pins. Internally, there are obvious differences due to the 9mm chambering and the lack of a buffer. Instead, the MPX-P uses a bolt carrier assembly attached to dual recoil springs. The bolt itself does resemble an AR’s closely with a cam pin and a removable extractor. It also rotates to fully lock like an AR, and this system provides lots of strength and safety in case of an over-pressured round or a bore obstruction.

The short-stroke gas piston system is also very interesting. Unlike a rifle system, the gas block is located far to the rear, just past the chamber. This provides maximum pressure from the fired round to ensure reliable operation even with heavy fouling. It also helps to reduce recoil and keeps most of the fouling out of the chamber and action.

Piston Performer

On the range, the MPX-P performed as expected, with zero failures of any sort, and I tested it right out of the box without performing any maintenance. Firing off-hand was a breeze with minimal recoil. The hand stop at the front of the handguard provided a good base for a push/pull technique that helped stabilize the pistol and keep it on target.

I tested the MPX-P’s accuracy at 25 yards by firing three different kinds of ammunition from a stable sandbag rest. I stuck with using iron sights only because I would for any other pistol test, but the MPX-P begs for optics, and I have little doubt that the performance would have improved with a suitable reflex sight. As it stands, however, there was little room for complaint, as the average for all five-shot groups was well below 2 inches.

From the benchrest, the recoil was not noticeably different than from off-hand, but there was a slight amount of trigger slap, which I only noticed from the bench. This was uncomfortable at first but was largely ameliorated towards the end of my shooting session—meaning it was likely just an issue of being broken in.

It’s quite obvious that Sig Sauer put a tremendous amount of forethought and design know-how into the MPX series. The modularity and versatility of the MPX is phenomenal, allowing a user to completely alter the weapon to suit their needs. I very much look forward to the availability of a carbine upgrade kit that would allow MPX-P pistol owners to easily convert the pistol to a short-barreled rifle.

RELATED STORY: We Are Legion: Sig Sauer’s Legion Series of Pistols

The select-fire version is intended to take on the dominance of the HK MP5 in this arena head on, and Sig may very well succeed. The MPX represents the next evolution of the submachine gun for military and law enforcement personnel. For civilians, it offers unprecedented options in terms of stocks, barrels and handguards with impeccable reliability and accuracy.

For more information, visit sigsauer.com or call 603-610-3000.