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Preview – SIG SAUER MPX – Next-Gen Sub Gun

Photos by Straight 8 and Kenda Lenseigne

And You Thought 9mm SMGs Were Dead…

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.


For the rest of this article, subscribe here: RECOIL Issue 21

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