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We’ve been hearing a lot about SIG SAUER’s new pistol caliber personal defense weapon, the MPX, ever since it was unveiled at SHOT Show a few years back. At the time the plan was to release both a pistol version of the firearm (with short barrel and no stock) as well as one with a permanently attached muzzle brake that would bring the barrel length to an ATF-appeasing 16″ in length. While the fate of that brake-festooned version is still in legal limbo, SIG is planning on finally releasing the pistol version to the civilian markets soon. We had an opportunity to get our hands on the gun and get some trigger time before anyone else, and boy is this thing a blast . . .

The gun was designed from the ground up to be a replacement for the aging MP5 submachine gun currently in arsenals world-wide. While the MP5 was a brilliant design when it came out in 1966, the state of the firearms design art has changed pretty dramatically since then and the gun now seems dated by comparison. For example, stamped sheet metal and riveted trunions are no longer staples of firearms design these days. H&K tried to update their iconic firearm by releasing the UMP, but its plastic-based design suffered from reliability and accuracy issues and hasn’t seen the widespread adoption its predecessor did.

The MPX solves these issues by using the overall design of the AR-15 platform and modifying the dimensions to better suit pistol caliber ammunition. This allows for a firearm that the end user in the field can modify it on the fly for whatever mission they are facing, makes maintenance of the firearms much easier, and cuts down on the overall size of the gun.

Here’s another issue inherent with the roller-delayed blowback system of the MP5 — you need to change almost everything in the bolt carrier when you swap between projectile weights. Moving from 115 grain frangible rounds to 147 grain hollow points? Break out the armorer’s kit. With the MPX, the gun just works no matter what you feed it. No changes required.

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SIG SAUER not only improved the operating mechanism of the gun, but also worked with Lancer to come up with a new polymer magazine design. Using the same tricks that Lancer uses in their AR-15 mags, they produced a lightweight, translucent magazine that has a steel insert that strengthens the feed lips and the catch slot as well, adding a ton of durability to the design. Oh, and they look bad-ass too.

Another benefit to the new mags is that the gun’s cyclic firing rate can be jacked way up thanks to their design. The version we played with was set to about 1,200 rounds per minute, which is well above the ~700 rpm rate of an MP5. The reason the gun is set so high is that the gun can feed reliably even at extremely high cyclic rates. The magazines are designed to present the next round almost directly in line with the chamber, something that the MP5 doesn’t do. So while an MP5 will start having serious reliability issues when running at anything over 1,000 RPM, the MPX has been reliably tested at well over 2,000 RPM.

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While the old MP5 design did come with a few options in terms of stock attachments, swapping one for another was a bit of a production, not something you could do quickly and on the fly. Thanks to the fact that the operating system of the MPX and MCX rifles are completely contained within the upper receiver, there’s no longer a need for a buffer tube or receiver extension. So SIG has decided to swap the old receiver extension for a Picatinny rail, and they’re making a boatload of different stocks that will attach to the rifle at that point. Everything from a side-folding stock to the nifty Honey Badger-esque sliding wire frame stock will attach to this short rail section. And because it’s a standard design, anyone can make alternate stocks for it as well.

Everything about this gun can be changed, from the stock to the handguards to the muzzle device. The current version of the gun sports a typical Picatinny rail system, but the R&D guys say they’ll probably be switching to the increasingly popular keymod system soon.

Out on the range, the MPX shoots like an absolute dream. Recoil is soft and straight, meaning that the gun is very easy to control even when you crank it over to the “auto” setting. The wire frame stock is actually pretty comfortable and the trigger is crisp and clean. The controls felt very familiar, just like the AR-15 we’ve all come to know and love.

The version we played with was the standard LE/MIL version, which SIG typically shows off with the removable two-chamber silencer. The small can might not seem very potent compared to the massive suppressors we’re used to seeing, but it’s so efficient that those two small chambers of suppression are all that’s needed to make the firearm hearing-safe at the shooter’s ear. It’s not exactly silent, but it’s damn near pleasant.

Thanks to our friends at the ATF, SIG will be shipping a pistol version of this gun (with SIG Sauer’s famous pistol arm brace as an option) to the civilian market and holding off on a rifle length version. Apparently the gun is designed to only use a short barrel, so making a proper 16″ barrel for the gun would take a bit of re-working in the operating bits of the firearm. But the pistol version will indeed be shipping shortly to the US commercial market. And when it does we’ll have a full review.

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54 COMMENTS

  1. So will the pistol version have the option for the infamous arm-brace using the rear rail?

    Hmmmm…

  3. I’ve said it once and I’ll say it again, the MPX is the Duke Nukem Forever of the firearms world.

    And I’m like the guy with the Game Stop pre-order from 2001….still waiting…..

      • That’s his point, I assume. The game spent a decade in development purgatory, even being rebuilt from the ground up a number of times for different console generations, and having the game engine changed three times. And the final product was awful.

        • oh man.. I remember fawning over the first demo screens in Game Informer (or some other gamer rag) in the late 90s with all my buddies, with whom I used to play Duke3d almost every school night and weekend over local dialup. The graphics looked so awesome at the time, when the game was planned to use the Unreal engine, and would be out “some time next year.”

          Cue to 2011, when one of those friends I still stay in contact with finally got the game, and I checked it out at his place… so disappointing, yet at least there was a sense of closure.

    • Awwww….you heathens you. Duke Nukem was so bad it was GOOD!

      I felt exactly like I was back in the 90s playing on my old IBM clone. With pixel graphics, cheesy midi music, and all those great one liners.

      Complaining about Duke Nukem forever is like complaining about a porno’s horrible plot.

      My only complaint about it was I never got to use a MP5 during the game.

      Hail to the king, baby!

  6. This looks like a nice piece if kit. However, I don’t see any benefit of the MPX when compared to a suppressed 300 AAC. A subsonic 9mm 147 grain doesn’t have much power. A .45 subsonic has more energy and momentum, but can’t penetrate body armor. The 300 had decent energy while subsonic, and supersonic loads can zip through Kevlar. Damn you Nick for making me buy another AR.

    Of course the 9mm pistol cal carbine would still make an incredibly fun range toy, and 9mm reloads are roughly the same price as .22 LR.

      • This response sounds like you think the civvie version will be full auto. Not a chance without repeal of some laws not even under discussion. If you’re a Class 3 dealer you can play with them, but they are for LEO only. The review didn’t mention that the versions we’re waiting for are semi only. But I betcha. Otherwise, that suppressed AR in .300 Blk can be select fire real easy, too! It’s just illegal.

        • that’s not what he meant; he means the semi-only version would simply be a range toy, though that’s still subjective. there’s plenty of practical use for a 9mm carbine.

    • Inside confined spaces with metal interiors like ship compartments and the like where a heavier bullet may have a higher chance of ricocheting. No experience, so I can’t say how realistic this actually would be, but it sounds plausible at least.

  7. Did you get a chance to look at the internals of the lower? Curious if it uses the standard AR LPK as the internals or something new for the MPX.

    • There was one rule that day: I was forbidden from opening it up. Something about an active contract bid for an undisclosed group of people who are very good at killing bad guys. Give us a month or two and we’ll have one to play with and take apart.

  8. There are a couple bits of information in this article that I’m not sure are 100% accurate from my experience with my MP5s.

    “Here’s another issue inherent with the roller-delayed blowback system of the MP5 — you need to change almost everything in the bolt carrier when you swap between projectile weights. Moving from 115 grain frangible rounds to 147 grain hollow points? Break out the armorer’s kit. ”

    I routinely shoot my MP5s (PDW and full size) with 115 and 147 grain ammo without changing a thing. Maybe I’m just lucky, but I’ve never heard of anyone having to swap out rollers unless they are attaching a silencer. Even then it’s not a happens every time to 100% of MP5s thing, just as a troubleshooting step.

    “While the old MP5 design did come with a few options in terms of stock attachments, swapping one for another was a bit of a production, not something you could do quickly and on the fly.”

    Changing the stock on an MP5 involves pushing two pins and putting the new stock on. It is, in my opinion, as easy if not easier than swapping an AR-15 upper. The only time I’ve heard of issues is with stocks that are not in spec.

    EDIT:
    Interesting, there must be another Brian posting on the discussion board. As that is not my picture, sorry to the fellow who is displayed.

  9. I’ve never seen a magazine wobble so much. Seems odd for something that can reliably feed at high rates – is there some intentional, designed, mechanism responsible for that movement?

  10. do want.

    but I’m a civilian, non Hollywood, non politician, in CA.

    I’ll live vicariously through you guys,

  11. Please dont be vaporware, please dont be vaporware, please dont be vaporware!!

    Might even do one of those shady preorder online deals…

  12. Why so high fire-rate? 700 is easily controllable. If they got it running reliably at 2000 why do they cap it at 1200? Sounds fishy.

    Also the article sounds as if written by someone without experience on the MP5.

    Sorry if I sound negative, wasn’t my intention.

    • Its called over-engineering. Cars, planes boats, even (some) toasters are designed this way, for reliability and safety. Anything that could kill you if it fails is generally over-designed.

      • I know about over-engineering I just see no point in bragging about how the gun can handle 2000 rpm then cap it at 1200. That is like making a car that can go 500 km/h but limit it to 300, yet still brag that it goes 500.

  14. I wonder whether the picatinny-compatible stocks will be sold to the general public. Imagine the convenience of adding a small rail section to the rear of an AK and using those nifty wire frame stocks.

  15. Me likes the MPX, pistol version with the sig brace, but, it’s MSRP is way, way too high for its intended audience. Be that police depts or civilians. Ther are far better priced options, that do the same thing as well if not better. You have to love the firearms industry, they make their regular customers, their beta testers, beta testers who pay full PT Barnum retail. I predict unicorn status for this weapon.

  16. The trigger looks a lot like an AR trigger from what I can see externally. Do you think you could drop in an aftermarket AR trigger group into one of these?

  17. I kinda want one of these, but I want a SIG 227 more, that or the carry variant, I’m not sure which yet

  18. Naturally, the only problem we have is the sorry fact that only the LE/MIL types get the full auto-fun package. So what we got is a review of a firearm that will never be available to be owned by most everyone reading this. Fan-fucking-tastic! It’s always a real hoot to see what kinds of weapons the LE community will use on us, not that any of them have proven that they are any more responsible or deserving of them than we are. Am I just in a surly mood, or am I the only person here that’s growing tired of seeing all the modern firearms that we’ll never get to own, let alone shoot, being reviewed by the special few who write gun reviews? What’s the point? So the 1% can tell the rest of us what a fucking great time it is to be among the 1%? Who gives a fuck what the goddamned cyclic rate of this Sig at FA is….if we’ll never get the chance to find out for ourselves? Gee, my heart’s all aflutter with rapt anticipation of the raucous, riveting review of the Sig MPX…semi-auto version. Who cares if the MP5 is old, or outdated, at least you can still actually buy one.

  19. Never had to switch a damn thing out on an MP5 to make it run with anything between 115gr and 147gr. Dropping the BS flag on that one.

    MPX looks nice. But, its just a pistol in the end. Yawn.

  21. Let’s see – 2,000 rounds per minute! That’s about 33.3 rounds per second. We’re getting perilously close to the “30 magazine clip that can fire 30 rounds in a half second.”

  23. Actually, in response to the comment about richocets in ship compartments, that statement is inaccurate. A heavy round will richocet less than a lighter, higher velocity round. The 9mm travels anywhere from 300 to 1200 fps(depending on the round used) faster than a standard .45, opening the door for a higher richocet probability.

  25. A $2K 9mm pistol that uses $65 magazines? They must have dreamed this up when panicked Americans were paying way too much for AR15’s. This is a cool concept but really…the SMG has been dead for years. The AR platform killed it. We sold off our MP5’s and went to short barrel rifles long ago. This is a gee whiz like to have it toy that will never be embraced by the general gun buying public in the United States anyway. Good luck Sig. This does nothing a G17 won’t do…except look cooler. And have a funky kinda stock thing.

  28. I saw something very disturbing during the slow-motion segment – note how the magazine is flexing/moving in the magwell – it looks as though it has a lot of play in it and perhaps the bolt/action is actually moving the top of the magazine forwards and backwards. Either that, or it was an optical illusion – like when you hold a pencil between two fingers and wave it up & down, making it appear to be made of rubber.

  29. A “high tech” blowback? Isn’t that a contradiction in terms? If you want a 9mm SBR I recommend a POF-5 with a B&T folding stock and a SureFire M63 rail.

  31. “Another benefit to the new mags is that the gun’s cyclic firing rate can be jacked way up thanks to their design. The version we played with was set to about 1,200 rounds per minute, which is well above the ~700 rpm rate of an MP5. The reason the gun is set so high is that the gun can feed reliably even at extremely high cyclic rates. The magazines are designed to present the next round almost directly in line with the chamber, something that the MP5 doesn’t do. So while an MP5 will start having serious reliability issues when running at anything over 1,000 RPM, the MPX has been reliably tested at well over 2,000 RPM.”

    Exactly how is this beneficial? Having a SMG that fires over 1200 rounds per minute isnt necessarily an indicator on the effectiveness of the gun. Most who have experience with full auto firearms will tell you once you start reaching above 1000-1100 RPM it drastically effects the accuracy and control of the firearm. Even in 9mm. There is a such thing as having too high of a rate of fire when it pertains to the control of the firearm. Especially when dealing with hand held guns and not ones braced by a bipod or mounted on a vehicle or other object. And 2000 RPM is just absurd and pointless. I have never heard of anyone complaining about the rate of fire for the MP5. Its perfect the way it is in the 800-900 rpm range.

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