The Ultimate Firearms Destination for the Gun Lifestyle

SIG SAUER MPX – Next-Gen Sub Gun

Like a character in a bad horror movie, the 9mm subgun seems destined to return time after time, despite every effort to declare it dead and buried. First, the intermediate caliber assault rifle seemed destined to replace it in every possible role. Then the micro-caliber personal defense weapon, or PDW seemed to have sunk the final nail in its coffin, filling its shoes when specialist troops such as vehicle crews whined that their carbines were too big to fit inside driver compartments.

Despite the very real advantages offered by 4.6 and 5.7mm rounds, the logistics chain simply didn’t need yet another caliber to source, manage, and deliver to the point of use, especially when 9mm Luger was already in the system and would do the 80 percent of the job, 90 percent as well. The Swiss, legendary spreadsheet experts that they are, were the most recent country to do the math and settled on B&T’s MP9-N as the sidearm for troops who didn’t need a StG90. They are reportedly very happy with them. (See RECOIL Issue 17 for insight). Similar to American experience with the M4 and its 5.56mm round, they’re good enough for government work, and any improvements gained by adopting a new platform and caliber would outweigh the arse ache involved in the switch.

It was against this background that SIG SAUER embarked on a program to develop the next-generation 9mm submachine gun, a position in western countries that’s been filled for the last 50 years by the Heckler & Koch MP5. With HK placing greater emphasis on their MP7 line and in-service MP5s racking up the miles, it made good business sense to offer an alternative for professional users looking to update their tools. Enter the MPX.



When it was unveiled at SHOT Show 2013, the MPX was the gun everyone was talking about. It then languished in development purgatory for 18 months, while engineers split hairs, marketing staff teased us with videos, and the shooting public growled, “Release the damn guns, already.” As you delve deeper into this neat little gat, it’s obvious that a great deal of thought and work has gone into it — according to SIG, their staff burned through 11 million rounds in R&D last year, mainly on the MCX and MPX lines.

We received one of the first pistol variants for T&E, but figuring that a lot of people are going to want to add a stock to this after getting their tax stamp, we also arranged for an SBR variant to make its way to into our grubby paws. Here’s what we found out.

The MPX differs from any other 9mm carbine currently on the market in that it’s gas-operated — a quick glance at a live round is all it takes to figure out why this might be a significant achievement, as there simply isn’t much in the way of powder (and therefore gas) to work with. Rifle rounds? No problem — there’s around 25 grains of powder in a 5.56 case to power the gun, but only 1/5th of that in a 9mm, which is compounded by much lower chamber pressure. To counter this, SIG moved the gas port closer to the chamber. How much closer? Well, an AR-15’s carbine length gas system uses a port situated about 6 inches ahead of the case mouth. The MPX’s lies only about 0.125 inches out.

Because the bullet barely has time to leave the case before the gas system comes into play, it’s very sensitive to changes in ammo. Or at least, it would be had SIG not incorporated a self-adjusting valve to keep operating pressures on an even keel. In order to test its effectiveness, we loaded up a mag with whatever random 9mm rounds had accumulated on the editorial desk: 115-grain American Eagle FMJ, 147 grain +P hollow points, a few 9mm Major reloads from the race gun elsewhere in this issue…you get the picture. Lighting them off proved to be utterly uneventful (apart from the running commentary of our training partner, which went, “…mild, mild, spicy, mild, habanero!”), with the bolt failing to outrun the mag springs and having enough velocity to pick up the next round, every single time.

For those concerned about a malfunctioning gas valve bringing the show to a screeching halt, don’t be. If it gums up with carbon, then a full gas charge hits the piston, rather than a metered charge — your gun will wear out a little faster, but it’ll continue to run. Due to minimal gas volume and pressure, care has been taken to keep as much of it inside the system until it’s done its job. If you think there’s something familiar about the gas plug and piston, it’s because both employ a one-piece McFarland gas ring from an AR bolt to seal the deal. Clever, cheap, and readily available should you ever need a spare.

The piston acts on a bolt carrier housing a multi-lugged rotating bolt, which has some nicely executed design features. Its bottom lug doesn’t lock up against a corresponding twin in the barrel extension, but instead is both enlarged and extended to push rounds out of the magazine and scoop them into the action. The four remaining bolt lugs appear to be overengineered for pistol-caliber rounds and are contoured in order to eliminate stress risers. An AR-style extractor yanks empties out of the chamber, while a replaceable AK-type fixed ejector kicks them out of the gun.

Covering the gas block and barrel is a removable rail section, which is retained by the front takedown pin and slides in grooves cut in the upper receiver extension. Adding a different length rail section (to cover a suppressor, for example) is simply a matter of pushing out the pin and sliding in the new one. Removing the handguard reveals a pair of Torx-headed bolts, which keep the barrel in place. Swapping barrels takes about two minutes, which is a good thing, as you’re going to get familiar with the process if you need to clean the gas system.

About the only niggle we have with the gun is that accessing the gas block is a royal pain, shrouded as it is by the upper receiver extension. The loud end is threaded 13.5x1mm LH to prevent dumbasses from screwing a .22 or .30 caliber can onto a 9mm, but which complicates matters for those of us who can think. We contacted our friends at Liberty Suppressors for an adapter for the Mystic X, and then sent a couple of mags worth of 147-grain subsonic ammo through it — compared to the tiny SIG SRD9 pictured on the cover, the Mystic X has about three times the internal volume and is ridiculously quiet. At 50 yards, impact noise of the bullet hitting the berm drowns out any report. This thing is a superb suppressor host.

One aspect of the MPX’s design that impressed us was the attention to longevity that SIG’s engineers have put into it. Its upper receiver is overbuilt (there’s an attendant weight penalty for this) and has steel reinforcement added in critical wear areas, such as the bolt cam pin track and charging handle latch points. A polymer buffer cushions the bolt carrier impact, and the whole thing feels like it’ll last at least a couple of lifetimes.

Anyone familiar with an AR-15 will be instantly at home with the MPX’s controls and manual of arms. Improvements upon Stoner’s layout have been made, and the gun benefits from an ambidextrous safety lever and magazine release. Bolt hold-open is left side only, though the carrier can be sent home using the right side bolt release. The AR’s charging handle is retained, and although at first glance this might seem a retrograde step, almost no one is going to ft a regular scope to the MPX, so ocular bell interference is not an issue. The trigger is likewise AR-based and features a block to prevent the hammer from smashing into the disconnector at the end of its stroke, which would not only shorten the service life of fire control components but also produce trigger slap. Unfortunately, the same attention to detail wasn’t lavished on the trigger pull, which at a gritty 8.5 pounds, sucks baboon ass. Self-contained triggers such as the KE Arms and Timney drop-in models are compatible, however.

Takedown is straight-up AR — cock the weapon, push out the takedown pins, and pull the guts out of the upper. On doing so, you’ll notice that the AR’s single recoil spring has been replaced by twin springs located on guide rods inside the upper receiver, which means there’s no buffer tube, and hence a folding stock becomes an option.

Two stocks will be offered from the factory. The first, shown here, is a collapsible model which is closely patterned on the MP5 A3 and which shares some of its shortcomings. To release the stock, the shooter presses a button on the receiver adapter and a spring-loaded plunger kicks the rods out to the first of two positions. To close it, press the same button, apply pressure to the buttplate, and at this point, the shooter curses the built-in sling loop for getting in the way. The good news is that you’re only one screw away from flushing the PIA sling loop and using the side-mounted flush cups or adding a wire loop sling mount.

Although it might look cool, the telescoping stock’s buttplate interferes with the shooter’s strong hand when collapsed and makes operating the gun more difficult than it should be. It also adds length to the package, so if your intention is to fle a Form 1 and SBR this sucker, we strongly endorse the side folding option.


We believe assessing a new design shouldn’t take place in a vacuum — you the consumer want to know how it stacks up against the competition. So let’s compare it to the most readily available alternatives. A Colt-pattern AR-15 9mm will run you a couple of Benjamins less than the MPX, is well supported by the aftermarket and offers the kind of modularity that only an AR brings to the table. Its magazines are readily available and can be had for less than half the cost of the (admittedly very sexy and reliable) Lancer hybrid stainless/polymer needed for the MPX. Being a blowback gun, there’s a big chunk of steel rattling around the upper at every shot as the bolt cycles, and this produces way more recoil than you might think possible from such a tiny round. As an adaptation of an existing design, rather than starting with a blank sheet of paper, reliability is not as strong as the alternatives.

An MP5 clone, such as the Brethren Armament BAP9 that graced the cover of Issue 13 is more expensive than the MPX, uses a proven delayed blowback system, and recoils about the same, which means it’s superbly controllable, especially in full auto should you manage to come up with the scratch for a transferable trigger pack. The MPX has an advantage in terms of ergonomics and niceties such as a full-length Pic rail, a last-round bolt hold-open, and better magazine release.

If we had any stoppages with this gun, we’d be the first to tell you. Accuracy ran around 2 inches at 50 yards with most ammo favors and everyone who shot it came away with a grin on their face. So what’s our overall assessment of the SIG MPX? After a couple of range sessions, we wrote two checks, one to SIG, and one to the ATF. And there aren’t many firearms that motivate us to do that anymore.



Caliber: 9×19
Barrel Length: 8 inches
Overall Length: 17 inches
Weight Unloaded: 5.2 pounds
Magazine Capacity: 30 rounds
MSRP: $1,575

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