A Semiauto SubGun? SIG Sauer’s MPX PSB 9mm – Full Review.

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The SIG MPX PSB is a pistol variant of the MPX series and features a “wrist brace” attachment.

SIG Sauer seems poised to take over the world these days. It seems like they have a winner in every market segment except kitchen sinks, and they are probably working on kitchen sinks right now. A huge win for SIG has been the MPX family of pistol caliber carbines (PCC), and this week we got our hands on one to test.

SPECS

  • Cartridge: 9mm
  • Barrel: 8 in.; 1:10-in. twist
  • Overall Length: 16.7 in. (stock folded); 24 in. (stock extended)
  • Weight: 6.1 lbs.
  • Trigger: 5 lbs., 8 0z. – 9 lbs.
  • Sights: N/A
  • Finish: Matte Black
  • Capacity: 30+1 rds.
  • MSRP: $2,084
  • Manufacturer: SIG Sauer

    The SIG Sauer MPX PSB ran reliably and functioned flawlessly with the SIG Elite Performance ammo. Photo Courtesy: SIG Sauer

Our version of the MPX was actually the MPX PSB (pistol stabiliizng brace) pistol version, which I guess makes it the pistol caliber pistol? The not an SBR but totally a semiautomatic 9mm sub gun? What the MPX represents is a leap forward in real sub-machine guns, with many flavors of semiauto, civilian-legal variations. The controls are all AR-15 in style, but this isn’t just another AR scaled to fit pistol bullets. The MPX was designed from the ground up as a new platform, with a retention of the design characteristics from an AR that it made sense to steal. This cuts the learning curve to zero, and also fits the MPX nicely into applications where a rifle is too much, but a pistol is too little.

Anyone familiar with the AR’s manual of arms will be right at home with the MPX series.

The firearm features ambidextrous controls for ease of use with either hand.

The pistol version ships with the ubiquitous SIG “Arm Brace”, with a slightly extended length from other versions I have seen. It is about 3 inches longer that the AR versions, classic SIG sticking it to the man every chance they get. Because the MPX doesn’t require a buffer, this brace isn’t also a buffer tube. More on that in a minute. The brace is attached to a folding hinge, which mounts on the MPX via a Picatinny rail on the back of the receiver. The folding hinge bolts to the Picatinny for a rock-solid joint. When you shoot the MPX with, uh, the arm brace velcroed in place, there is no movement in the stock, which is awesome. The hinge is a little tight, it takes some force to unlock, but it works great.

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The receiver set looks very similar to an AR, and it is fully ambidextrous including the charging handle. Your welcome, wrong handers. The magazine release button has gotten a little redesign, it is about twice the size of a normal AR-15 on the right side, and the paddle on the left is about the size of a regulation normal one. The safety selector is AR similar on the left, and little shorter that your old familiar controls on the right. The left side bolt release has an added bit of length to the bottom, making it ¾ inches wide, very easy to press for locking the bolt back. The right side bolt release is absolutely massive, and you can easily reach it with your trigger finger. Only the left side will lock the bolt to the rear, but both sides release it with ease. The magazine well is slightly flared, which really helps facilitate quick reloads. The MPX runs on a proprietary Sig magazine, which gave me zero problems. The magazine is clear plastic with steel feed lips, and runs buttery smooth. Loading the magazine is easy, no Uzi magazine problems for this little guy. We only had one magazine for testing, the factory standard 30 rounder. I was very happy with how easy the magazine came apart for the installation of my Taran Tactical Innovations +10 extension. TTI had just shipped the new aluminum extension in Coyote Bronze, and it ran like a champ. This extension does not require a new spring, which also tells me SIG was serious on spring tension. Adding 10 rounds is no small feat, so the factory 30 round was absolutely meant to work in all conditions, for a very long time.

The MPX PSB is a compact and capable firearm and feeds from a proprietary magazine system.

The author equipped the magazine with a Taran Tactical Innovations (TTI) magazine extension to bring total capacity to 40-plus-one capacity.

The upper and lower come apart with just like an AR, with a takedown pin front and rear of the receiver. This is where the similarities end though. The MPX doesn’t use a scaled-down AR bolt, though both are cam based locking with rotating lugs. The MPX has two springs on top of the bolt that recoil against the receiver, and push the bolt back into the chamber. This cuts down on the overall size of the bolt, and also means the MPX works normally with the stock folded or taken off in the pistol variant. No nubby buffer tube required. At the back of the receiver is a large polymer ring, which absorbs the impact of the bolt, and keeps your steel bolt from smacking the aluminum receiver.

How It Works

The action is neither direct impingement or blowback, it is a short stroke piston system. This keeps most of the filth from pistol rounds out of the receiver and has proved to be very reliable. The barrel on the pistol version is 8 inches, but has the new MPX quick barrel change system just like the carbine. If you push out the front takedown pin, the handguard pulls straight off. This exposes two hex screws, pull those out, and the barrel comes right off. This makes barrel changes a snap, and in the future caliber conversions will be easy as the day is long. One of my only complaints, SIG put a 13.5×1 metric thread pitch on the barrel. If you use direct thread suppressors, better make sure you can get an adaptor.

The MPX doesn’t ship with iron sights, but this is a pretty easy fix. We are talking about SIG after all. For testing, we were also provided a Romeo5 1x20mm compact red dot sight. This optic features a 2 MOA red dot with 10 illumination settings, 8 for daylight and 2 for night vision. This sight also features MOTAC, motion activated illumination system, which powers up when it senses motion, and powers down when it doesn’t. The dot is very bright; I had no trouble finding a setting for broad daylight. The controls are a bit different, but it worked just like an Aimpoint. In the box are mounts for both low and 1.41 inch co-witness bases, which is a nice touch. I liked the high mount for the MPX pistol, and will try the low mount next week on a rifle 45-degree offset.

The author also had an opportunity to try out Sig’s air gun variant of the MPX as well.

A Little Extra

In keeping with the SIG complete package, what could possibly make this any better? How about a training tool to go with it? Also available from SIG, we received an MPX airgun and target system. SIG has really come out swinging on this front; they have introduced several airgun systems over the last year. The MPX air comes with a real buttstock since it isn’t a firearm, but is otherwise identical to the MPX pistol. The airgun runs off of CO2 cartridges and fires .177 caliber steel pellets. This is no child’s toy, as the pellets reach up to 600 feet per second (fps). It even comes with an incredibly similar red dot sight. Big applause to SIG Sauer on this one, replica airsoft or pellet guns are a fantastic training tool to round out a system. The savings in ammunition cost are staggering, and it isn’t hard to set up an airsoft range in your garage. Can’t say the same for 9mm, that is a certainty.

Note the ambidextrous AR-style safety lever and the enlarged trigger guard.

Conclusion

This is the first time I have ever had optics, gun, and ammo all made by the same manufacturer, which is pretty wild to consider. If I had a better license, I could have gone ahead with a suppressor as well. A training replica gun is pretty awesome as well, and pellets are arguably a better option than a .22 LR conversion.  Have you tried to buy .22 recently? The MPX ran like a sewing machine and had zero malfunctions. Magazines aren’t cheap, but they are available, and it’s nice to have aftermarket support already from places like Taran Tactical Innovations. A 40 round sub gun magazine is pretty hard to argue with. There was a lot of winning with this system. The SIG 115 grain ran like a champ.

The SIG Sauer pistol stabilizing base allowed the author to accurately engage steel targets and allowed for fast follow-up shots. Photo Courtesy: SIG Sauer

So what was the bad? I only have a couple of complaints. One is the trigger, which is less than good. To be fair, it is about like a Mil-Spec AR. The big problem is a replacement. I don’t believe in bad triggers, especially after I pay $2,000 for a gun. AR-15 triggers fit the MPX, but SIG says the new gun is very hard on fire control systems.

Secondly, I would have much rather had this gun for testing in the carbine variant. I understand why SIG makes a pistol version, some people are really in love with this and don’t want to SBR. But for a shake out of a weapon, it is a lot easier with a real stock and a grown up barrel. A 16-inch carbine version is on the market right now, and that is a much better choice for most consumers. Even in the pistol version though, I would have preferred to see an 11-inch barrel. This has nothing to do with velocity, and everything to do with the hand stop. On the 8-inch barrel, even with the hand stop installed, it is entirely too easy to get a finger near the muzzlebrake. I have seen this happen with an MP5, and the result isn’t pretty. If you buy the pistol version, be extremely careful.

Although the author found the 8-inch barrel a little too short for his taste, the barrels are user changeable for in-field adaptability. Simply remove the barrel and A2 muzzle device and swap for the desired length. Photo Courtesy: SIG Sauer

For more information, visit https://www.sigsauer.com/store/sig-mpx-psb.html .

To purchase an MPX on GunsAmerica.com, click this link: https://www.gunsamerica.com/Search.aspx?Keyword=MPX.

{ 30 comments… add one }
  • Russ H. July 18, 2017, 11:20 am

    After reading all the dumb comments here I\’m going to go out and buy two MPX\’s just because. Those comparing Uzi\’s and Tec 9\’s to this gun – LOL!!!!! I\’ve carried and fired (a lot) the Uzi and MP5 (full auto) variations and love the MP5 family. Yes, I love the MP5\’s but I\’ve seen the MPX for $1000 (ok, just once, $1300 a few times). Meh on the Uzi and LOL to the Tec 9. Tec 9? Haaaahahahahahahahahaaaaa!!!! Really? Love, love, love Sig for what they\’re doing – waiting for a semiauto shotgun to compete with the Benelli. Sig has come a long way since I bought my first P220 45 back in 1988.

    • Ed July 20, 2017, 8:50 am

      Welcome as a member to this “dumb comments” column. You’ll receive your certified “I Post Dumb Comments” certificate, shortly.

  • Prepperdaddy July 12, 2017, 4:25 pm

    Please let me know when the 10mm version comes out.

  • Ed July 12, 2017, 1:54 pm

    Well boys, I have an MPX and I like it. Function is great, shoots good and I would highly recommend it. So regarding disparaging comments made comparing it to the Uzi and MP5, let me give my personal comments. Both the Uzi and MP5 are great guns which have proven themselves time and again in actual fights. As a civilian user however, here are the downsides of the Uzi for me, 1. rear sight is a dinky little hole to try and locate so you can picky up the front sight (not at all like the Garand sight which is easy to pick up from the get go), 2. It’s heavy-it’s all pressed steel and in semi-auto has a big, heavy bolt to retard the recoil of the 9mm. Having said that, I like the Uzi-just recognizing it’s “issues”. Regarding the MP5 the downsides for me as a civilian user are, 1. Price-the clones run $500 more than the Sig,, 2. It doesn’t have a bolt hold open after the last round, and 3. It chews up brass if you reload-which I do. I like the MP5, just like I like the G-3 and the G-33 but they chew up brass so much because of the fluted chamber, that anymore I just shoot steel-case ammunition. I took all of these points into consideration before ordering an MPX and I’m pretty happy with it and believe others will also. Yes, with the short barrel you better watch where you position your front hand-on any of them.

  • MOS8541 July 11, 2017, 4:21 pm

    LOL, love it, a “wrist brace”. Aaaanny way sig doesn’t have for example an automatic shotgun to compete with Benelli, etc. I have owned 550s for years,which are all better than these mxs which are basically updted AR18s.

  • JoshO July 11, 2017, 12:47 am

    lol…Sig Sauer still exists? Why on Earth would you be a beta tester for this junk when you can have a semi-auto, brand new, tried and true through over 50yrs in the field MP5K for the same price?

  • RETSWATCOP July 10, 2017, 10:40 pm

    Carried a “Navy lower” MP5 while “in the stack” for 12 years then we changed to UMP45s. Bought the transferable MP5 from the PD when it retired and have not found any of the fake SMGs to perform as well or be as durable. Real Uzis, standard and mini, are nearly indestructible too

  • beau rivage July 10, 2017, 7:14 pm

    Dear, American I’ve not had the oportunity to test the SiG Sub gun….but I do know the derivation or to be more accurate the etiological basis, for the Colloquial Referenced Sub Gun; Sub gun refer’s to weapons that fire a Sub-Caliber or Pistol cartridge..22,.380,.9mm, .45 acp etc. AS opposed to a Rifle or Major Cailber .223,5.56, 7.62 x39 , .308 , 30.06 ….etc while not all inclusive the major and most popular American Based Cartridge /weapons platforms for a demonstration of my point. While it is true most often that weapons are developed first for military application, then if a civilian market is identified for a civilian distribution secondly but Full auto wasn’t something the military ever really embraced, in the past 50 yrs. for instance, there were numerous training programs in the 60’s 70’s80’s and 90’s that taught semi auto or burst fire techiques. One to conserve ammo and increase hit probability.But for my money area denial and for keeping the bad guys heads down.! Full auto is great but …when its time to get the “F” out a dodge or when it is time to close the deal ,FA is the way to go… have one and all

  • Stef July 10, 2017, 3:33 pm

    When make a gun and charge $2k with a crappy trigger. So it’s what everybody wants, I’m not interested in a pretty gun that doesn’t function well. I mean come on, us shooters use the trigger more than anything else in a weapon…. shame on you sig…

  • Wetwrks July 10, 2017, 12:34 pm

    I am a Gen1 MPX owner. Never had any trouble with my gun…but that is not the case for a lot of the buyers. Yes…there is already a Gen2 MPX on the market in an attempt to correct some of the problems found with the Gen1. This has in turn created other problems. Mags are different between the two generations and won’t always work from one to the other. You mentioned the AR trigger and fire controls…while aftermarket fire controls will fit, they are not recommended and most of those manufacturers will not warranty their product if put in the MPX. It has an additional “ramp” in the MPX fire control that keeps the bolt above the fire control that is unique to the MPX. The AR style charging handle is proprietary to the MPX…you cannot use any other AR charging handle. There seems to be a growing industry of little shops ramping up to make their own copy. While the magazines are well made, you pay for the quality. I have found them as cheap as $30 but typically they run $70 per mag. Lastly…as for the quick change barrel…well…that so far (after several years on the market) has proven to be vaporware. There are no additional barrel lengths available from Sig nor are there any available caliber conversions available to be purchased by current owners. As of the last update I have seen there is also no ETA for these highly advertised barrels and conversions. As I said at the start…I like my gun. Would I do it again…most likely. Am I disappointed with Sig…definitely.

    • MOS8541 July 11, 2017, 4:25 pm

      What I want to know is why we cant buy FA class 3 whatever New? That law graandfathering transferable dealer this sample that,are stupid.

      • Dewey July 11, 2017, 11:24 pm

        Because the REPUBLICAN president, Saint Ronald Reagan, signed the McClure-Volkmer act into law. That’s why.
        I don’t expect or want you to believe me, look it up yourself. Look up how many REPUBLICAN presidents infringed upon our second amendment rights and maybe the left/right, R/D paradigm will become clear.

  • American July 10, 2017, 11:50 am

    A SEMI AUTO sub gun ? It’s either a sub machine gun or it’s not .. The term sub machine gun came about because they were smaller than a machine gun. It’s not a sub machine gun unless it fires full auto. Need real information not sales gimmicks .

    • kane July 10, 2017, 3:08 pm

      Yes, a SEMI AUTO sub gun that fires a pistol cartridge, it’s not a “sub machine gun.

  • Norm Fishler July 10, 2017, 10:50 am

    I went through this decades ago with the Tec-9 multiple times. My excuse back then was that I was young & dumb. Dunno what I could use for an excuse these days. The Tec-9 was considerably smaller, lighter & handier than this SIG clunk you’re promoting here & I cannot help but wonder why you even make an issue of its existence. The Tec-9 was, as I recall, reliable and within the bounds of reason, accurate enough for its intended purpose, which I always considered to be a gun fight in a phone booth. If you GAVE me the SIG I would probably take it, & I might even keep it . . . for a while anyway. But without the full auto capability it remains as Jeff Cooper used to say, ‘an answer in search of a question’.

    • Alan July 11, 2017, 9:00 am

      You must be one of a million, The Tec 9 was a P.O.S., plain and simple.
      I had extensive experience with them at my gunshop.
      Of the dozens I sold, NONE worked well out of the box, MANY required extensive work to make work, and I never saw the value in a heavy pile of excrement that shot so poorly.
      But they were so popular, I helped develop a ‘no gunsmith’ mount for lasers that we marketed briefly.
      To call the Tec 9 reliable is a massive stretch.

      • Norm Fishler July 11, 2017, 4:47 pm

        Alan,
        I had 3 (three!) of them over the course of about 8-10 years & every one of them worked and ran reliably without any aftermarket parts or gun smithing. I have known of and shot perhaps another four and they too ran just fine. The lone exception was a very early one that literally disintegrated in my hands before I could finish the first magazine. But then I’ve had that happen to me with Colts too. Believe me when I tell you that I know what you’re talking about. Granted it is not an HK, but it filled a niche in the shooter’s lineup and they remain popular as a home defense weapon to this day.

        • Alan July 13, 2017, 11:14 am

          I believe you, I’m sure after awhile they worked out the bugs.
          As a point of interest in this discussion though, it was the Tec 9 that made the Indianapolis P.D. revamp their Beretta’s to DAO after a Patrol cop shot the hell out of a Perp and his car car when the fool pulled a Tec 9 and shot at the cop.
          The Tec 9 was found next to the Perps body, jammed from a failure to eject completely.
          The cop was so scared he just kept pulling the trigger, missing about half the mag, so both Perp and car were shot up.
          The cop was lucky. The poor shooting was blamed on the Double action of the Beretta.
          Guess the Perp had an early version Tec 9.
          After that, the I.P.D. went to a different gun, as the DAO conversions weren’t all that great.
          I sold one of those stamped “IPD” guns that later was used in a murder in L.A.
          The L.A. P.D. blamed it on the Indy cops, for auctioning their guns.
          What a joke, it turned out to be a ‘straw’ sale of course.
          That had the Lefties up in arms!

  • Richard Downing July 10, 2017, 10:46 am

    All the negative comments are from the plastic gun boy’s!

  • Tripwire July 10, 2017, 10:18 am

    No love for this one. I bought a PSA 9mm PCC and love it, I’ve abused it since round one and have yet to have the first malfunction. Cost…$650.00 and uses Glock mags! Win Win!… Awaiting the PSA in .40.. and of course 10MM.. and in the end when they come out with them I’ll have all three for less than this SIG thing… But for the folks who have to have every new shiny that comes out, go for it!!

  • Nathan Farnsworth July 10, 2017, 9:40 am

    I’m not sure what is to get excited about here. SIG develops a PCC (Pistol Caliber Carbine) that has 40% AR flavor and it’s taking over the world? Please. The brace tells me they are still marketing to those who think skinny jeans are okay. The AR-style charging handle on a PCC is idiotic, but I guess some folks won’t understand that. Ultimately it’s Heckler & Koch who did everything right with the MP5 variants and the UMPs. My SP5K and my UMP45 are flawless, a design tortured in the field and I don’t have to worry about SIG’s insane marketing promises, fads, design changes, and unavailability of parts.

  • Cyrus July 10, 2017, 8:27 am

    Stupid and useless in my opinion

  • roger July 10, 2017, 7:49 am

    Franklin is coming out with a Binary Trigger to fit it. Should be sweet.

  • nick mascari July 10, 2017, 7:04 am

    I’ve had my sbr for a year and it one of my favorite guns to shoot . Added the Franklin bfs binary, and 3 lug for my silco octane , aimpoint …so much fun . I’ve used a surefire Ryder on it as well with almost zero blow back

  • Rich W. July 10, 2017, 6:27 am

    For as much as they cost, they could have put the newer PDW style brace system that SB Tactical makes on it, instead of the crappy SB-X on a folding mechanism. It would become just as compact and be more stable, as it has 3 locking positions like an AR-15 collapsible stock. At that MSRP and with that crappy 1st Gen brace, I’m not a buyer.

  • Matt July 10, 2017, 4:40 am

    I absolutely love mine, and purchased it expressly for suppressing it. Buyer beware, there are a lot of reports of gas blowing through the charging handle area and back in your face when shooting with a can. And mine is no different. No fun at all getting pelted with hot gas. That said, from what I have been able to find, it happens on some guns and not at all on others. Something to do with which suppressor you are using. Last I looked there was no Gas Buster charging handle available yet, so mine is in the safe until one becomes available.

    • Rich W. July 10, 2017, 6:31 am

      You could probably use the automotive gasket maker trick that works for suppressed ARs to make your own gas blocking charging handle. There are a few different tutorials on YouTube that show you how and it’s very inexpensive and easy to do.. I did it on my AR pistol and I don’t even run a suppressor.

    • Steve July 10, 2017, 8:09 am

      How can you love it if you can’t even shoot it?

    • Andrew reeves July 10, 2017, 8:10 am

      Bought a psb also too, gen 1 though. 1300 with the black case, extra mag, NIB applied for my form 1. I use my gentech tundra, with thread conversion. Will also get the telescopic shoulder brace after the form 1 clears. Love the gun and added a varriant of the forearm left hand barrel hold. Enjoy yours.

      • steve July 10, 2017, 1:57 pm

        why would you get a brace instead of an actual stock if you are going to form 1 it?

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