Review: Sig Sauer MPX-PSB 9mm

There has been an explosion of advancements in the firearms industry over the past decade. In fact, this explosion of advancement has led to the popularization of many old concepts that have been modernized and made new again by the internet, such as the short stroke gas piston, the bullpup rifle, and the pistol caliber carbine. I was bewitched by the short stroke piston driven AR-15 in early 2007, and I have yet to go back. I find the luxury of abbreviated cleaning sessions a bit too nice to ever go back to a direct impingement AR-15. As for the bullpup, I’m still waiting for the Desert Tech MDR to come out so I can get my bullpup fix without having to compromise my trigger-hand ergonomics (you know, like magazine release buttons being activated by the trigger finger, etc.). And although there have been pistol caliber carbines out in the market forever, there has been a demand for an AR style carbine designed specifically for handgun cartridges. The problem with these carbines is that the majority of them are straight blowback. This isn’t necessarily a problem, but I don’t like straight blowback guns. Why? Because they are dirty… And they require a stupid heavy bolt creating more recoil than necessary… And if you shoot them suppressed, you usually get great deal of gas in the face due to a longer dwell time. This is where the Sig Sauer MPX comes into play.

The MPX is a firearm that fills the gap between pistol caliber carbine and submachine gun that the public has been wanting for quite a while now (we will ignore the fact that semi-automatic clones of the MP5, UMP, and other submachine guns have been on the market for a long time, because we won’t taint this review with your ‘facts‘). The appeal for the MPX is not that was developed first as a submachine gun, or that it’s an AR style platform. The appeal manifests in that it is both of those things AND it’s piston driven! No more gas in the face, no more finely machined rollers to wear out, no more heavy ass bolts to make me angry; just honest to goodness piston driven pistol caliber deliciousness. So, enough blabbing about nonsense and let’s get to the review!

A buddy of mine picked up an MPX-PSB (the pistol version with the SB-15 Stabilizing Brace) in early May of 2015. When I first laid eyes and hands on the MPX, it was a mix of emotions. I felt excitement, arousal, and lust… that is until I tried the trigger. This trigger is, by far, the WORST trigger I have ever felt on a gun. Now, I will remind you that I sell guns as a sales associate at a sporting goods store on the weekends and I have felt some bad triggers in my time; the most notable being in stock Bushmasters, stock LWRCI M6 carbines, and anything that is made by Kel-Tec. The stock MPX trigger reigns supreme in terrible triggers. To be fair, I have felt other Sig Sauer AR variant triggers, and they are all fairly decent for ‘mil-spec‘ triggers. So considering my unbiased opinion, I have NO IDEA where Sig is getting these MPX triggers from. The original factory trigger felt like a 4-stage trigger (you know, because it had four breaking points, all with different weights, but only the last one dropped the hammer) and made the gun a bit of a chore to shoot. This killed some of the magic for me, and the MPX went from a gun of lust to just another gun on the rack.

Fast forward to June 7th, 2015. I finally get to shoot the MPX at the range; only my buddy swapped the stock trigger for a 3.5 pound CMC flat-face trigger. I had never been a fan of the flat-faced triggers prior to shooting the MPX, but the experience I had at the range changed my opinion considerably. With the new CMC trigger, this gun is not just nice, it is exquisite. The MPX went from a feeling like a regular car with a sporty body kit, like what teenagers do to Toyota Celicas, to feeling like a genuine sports car with a finely tuned engine. The gun pointed easily and just about shot itself. Once I got it dialed in, anything within 25 yards was child’s play. The gun has a fairly strong recoil spring assembly, so the gun recoils very gently. I won’t liken it to a .22 LR (because it’s not), but I will say it feels different from anything I’ve ever shot before. Since the CMC trigger was so nice, I was even able to bump fire it while looking down the sights a few times (and boy howdy, does it have a fast rate of fire – I didn’t demo a fast burst in the video).

Screenshot_2015-06-08-12-39-09

The only issue I had with this gun is that it begged and pleaded for a stock. The lack of a stock to stabilize the gun made shooting out past 25 yards without an optic pretty difficult. Moreover, the SB-15 brace that comes with the gun is a siren’s call; one that might get me in trouble. The distance from the butt of the gun to the strap portion of the stabilizing brace sits at exactly the distance I pull my personal guns to my shoulder. Thus, I might find myself throwing it into my shoulder and committing a felony without thinking about it. And as silly as that sounds (and as easily people will scoff at me following the ‘Thou Shalt Not Shoulder Thy Brace’ commandment), I’d rather not tempt fate by walking that line of the law. Holding the MPX without anything to brace upon, however, is a lot like trying to dance with that cute girl from high school, but you have to leave at least a foot of distance between the two of you for Jesus (and you know how bad you both just want to squeeze in close). In addition, the gun is awkwardly heavy when held out like a handgun, so I asked my buddy to remove the SB-15 brace and put a foam wrap around the tube. This helped me substantially, because even though there was no stock, I still had something to brace my cheek against to line up the sights properly. Regardless, I still wanted to shoulder it SOOOOOO BAD. And for this reason, I will most likely donate some money to my buddy so he can file an ATF form 1 and make the gun complete.

CIMG2160

For accuracy, the firearm is capable of remarkable accuracy. With 5 rounds I only managed to make a roughly 1.25″ group at 25 yards with iron sights (picture to the right>). However, with a stock on it, I imagine this gun is capable of single hole groups at 25 yards, and sub MOA at 50. Hell, I’d bet a real marksman could make sub-1″ groups at 100 yards with this gun wearing a proper stock.

I suppose one other gripe about this gun has to do with the safety levers. There doesn’t seem to be a way around the safety digging into your trigger finger when you fire the weapon. There are safety levers on both sides of the gun, so when you shoot, the recoil makes the safety lever dig down into your trigger finger. I didn’t notice it until it my finger began to get tenderized by the safety lever, so it doesn’t really weigh very big in my mind. However, it is there (a lot like that creepy uncle that shows up to every family reunion) and may impact how people feel about the gun. I guess if you shoot it enough, you’ll grow a callus and it may not bug you as much.

Range time information:

Firearm: Sig Sauer MPX-PSB
Magazines: 30 round (x2)
Caliber: 9mm Luger
Ammunition: Sellier & Bellot 115gr FMJ (my personal favorite 9mm ammunition)
Temperature: 65 Degrees Fahrenheit
Elevation: Somewhere between 50 to 100 feet above sea level
Total Rounds spent in session:~200
Final Disposition: LUST

PROS:

  • Compact design and excellent ergonomics
  • Can take any drop in AR-15 Trigger (a light one will allow bump firing) – Only downside is Sig Sauer announced that changing the trigger out will void the warranty… TOTAL BOLLOCKS
  • The gun is remarkably accurate with very little recoil
  • Its short stroke piston is cleaner to shoot than traditional straight blowback (is it?)
  • 90% ambidextrous (you forgot the charging handle, Sig!)
  • Not a single failure
  • Magazines are easy to feed
  • It looks effing sexy

CONS:

  • It’s expensive ($1500 on average)
  • Only comes with ONE proprietary magazine (dick move, Sig)
  • Needs to be SBR’d to obtain perfection
  • Ambidextrous Safety digs into your trigger finger when firing
  • It is a little heavy (I need to work out more)
  • 13.5×1 LH Muzzle device threading (WTF SIG? 1/2″x28, 5/8″x24, or GTFO)
  • Shooting it is addictive (goodbye ammunition)
  • Causes spontaneous chubbies and may cause wet dreams
  • May impregnate your daughter

I have shot SCARs, high end ARs, AKs, PS90s, Benellis, and other high quality firearms, but none of them give me the same kind of feeling that I get shooting the MPX. There is something about this gun that speaks to me deeply, and I may not be the only person to feel this. If I spent more time with it and it continued to provide the same accuracy and reliability that it showed at the range, I would have no qualms about trusting this weapon with my life.

To conclude, the MPX-PSB from the factory is okay, but with a good drop-in trigger, it is exquisite. I’m not going to lie, I did not want to stop shooting it, but seeing that I had gone through half of my S&B 9mm reserves shooting this gun, I had no choice but to stop. The gun is a little hefty, but it is solid. It will plead and beg for you to put a stock on it, so factor in another $200 for an ATF form 1 right when you buy it. Magazines and parts are still just about impossible to find, but there are hidden gems all over the internet with magazines available (just be prepared to fork out $70 for one). Sig Sauer has been running a promotion for MPX buyers that include three extra magazines, a magazine pouch, range bag, and 20% off of a Sig product from the Sig Store. They will ask for a serial number, so don’t expect to bamboozle your way through this promotion. And regardless of what history says about a lot of newly developed firearms, I suggest you get comfortable looking at the MPX; it’s a solid gun and it’s here to stay.

Sitting on the bench
Sitting on the bench

Loaded mags are happy mags
Loaded mags are happy mags

CMC Trigger sitting in the MPX
CMC Trigger sitting in the MPX

What the bolt looks like after 200 rounds
What the bolt looks like after 200 rounds

She's a dirty girl
She’s a dirty girl

Little bitty Charging handle
Little bitty Charging handle

MPX Bolt Face
The MPX’s bolt face

Have to remove one of the guide rods to disassemble the bolt carrier group
Have to remove one of the guide rods to disassemble the bolt carrier group

You can see this buffer takes a beating
You can see this buffer takes a beating

MPX Firing pin and bolt
MPX Firing pin and bolt

Another view of the bolt
Another view of the bolt

Cut in the bolt carrier for the ejector
Cut in the bolt carrier for the ejector

Underside view of the ejector
Underside view of the ejector

Port view of the ejector
Port view of the ejector

The inside view of the ejector
The inside view of the ejector

CIMG2171
Inside the gun from the back/ look at the piston op rod

This gun gets pretty dirty for a piston gun
This gun gets pretty dirty for a piston gun
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9 thoughts on “Review: Sig Sauer MPX-PSB 9mm

  1. Interesting review other than the sexual references; could you post a video of bump firing ? My team is looking for a pistol caliber sbr and this sig mpx could fit our needs if it can go “hyperactive” with a trigger change and a bit of practice. Agree of the odd thread size as this leaves us in a challenging spot regarding a suppressor, do you have any suggestions for the mpx ? A pistol caliber, piston driven suppressed sbr in a reasonable price range is very desirable for hostage situations, close range house to house searches etc. that my team encounters on a ever increasing frequency.
    More information, please.
    One more thing: why does your blog start with “derka derka” ?

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    1. Thanks for reading. I’ll see if I can post some more video in the next few weeks. And as for hostage situations, what exactly do you do for a living? You don’t work for one of America’s largest malls, do you?

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    2. Regarding threaded muzzle devices, Silencerco has a piston for their Octane HD and Osprey suppressors that has a 13.5x1LH thread pitch. To use one of those suppressors on the MPX, you will need to buy the piston and a separate spacer that Silencerco makes (which will replace the spring on the piston).

      “Derka Derka” can be used to describe the kind of AK-47s found in the Middle East. A quick google search will tell you what exactly it is.

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    1. I have read the National Firearms Act of 1934 and I understand it primarily refers to interstate commerce, but I have also read the BATFE’s ruling on the Sig Arm Brace and know they think it’s a felony if I ‘re-purpose’ it to my shoulder. The Bureau of Alcohol, Tobacco, Firearms, and Explosives has the power to invent and enforce any regulations they want with just about unlimited government backing, so I won’t speculate what they may or may not do. I love my right to possess and use firearms, so I’m not going to risk losing that right (on top of a $100,000 fine and 10 years in prison). The law is stupid, but we are stuck with it, and regardless of how we feel about it, the government will still enforce it as it sees fit.

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  2. Great review! I am new to guns but couldnt resist purchasing this one. I really want to lighten the trigger up, could you please provide an exact model # for the one you used? Seems tedious but I dont want to order something I cant use and my lack of knowledge would likely result in exactly that. I am very interested in the silencer as well, if you have any links to the parts I would need that would be awesome. Or of course you can tell me to get bent and do my homework but remember, everyone hates homework. Happy shooting!!

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