In 2013, SIG Sauer set out to design and manufacture one of their most ambitious creations – a gas-operated submachine gun, chambered in 9X19mm. A new generation of MP5-style guns, if you will. SIG’s team was successful in their endeavor, and after many tests and trials, the SIG MPX was finally introduced in 2015.

Here are some of its specs. For this review, we will be discussing the MPX in its pistol form (MPX-P). This firearm has several variants, and we will mention some of them later on.

SIG Sauer MPX Specs

  • Caliber: 9×19 Parabellum
  • System: Short-stroke gas piston, semi-automatic
  • Overall Length: 17″
  • Width: 2.35″
  • Height: 8″
  • Barrel Length: 8″
  • Weight: 5 lbs
  • Capacity: 30 Rounds
  • MSRP: $1852
Sig Sauer MPX K

SIG Sauer MPX Design

This thing is built a lot like an AR, and that is no accident. The AR-platform is one of the most recognizable and widely-used designs in modern firearm history. The bolt catch, safety button, and magazine release are all styled in AR fashion, and to make it even better, SIG has made this platform completely ambidextrous. I say “better”, though to be fair there are those who are not too fond of ambi-controls. This thing has a fully closed and locked rotating bolt mechanism, which means it can a lot of punishment and stay operational, without malfunctioning for even a split-second.

The MPX has a full-length Picatinny rail on top, and it comes with flip-up iron sights. There is also a KeyMod rail on the forend. The rear of the gun also has a small Picatinny rail, allowing you to mount the stock or brace of your choice on the back. Some attachments require paperwork, so make sure to comply with your local laws and regulations. The MPX’s grip is made of polymer.

The place where the MPX varies greatly from the AR-platform is its operating system. Instead of an AR-like direct impingement system or MP5-esque blowback system, the MPX utilizes a short-stroke piston system, and this is part of what helps the gun be much safer, much cleaner, and much more reliable than similar submachine guns.

Sig Sauer MPX PCC

SIG Sauer MPX Performance

The MPX looks great and also shoots great. The 30-round magazine is plenty, and it is an accurate and precise weapon right out of the box. The MPX is meant to be a close-encounter personal defense weapon, and it does its job in the best possible way.

The one thing I saw as a shortcoming – one which many will agree with me on – is the factory trigger. It is not the best, to say the least. I am not such a picky trigger squeezer, normally. That means that if I noticed its choppiness, you can bet your bottom dollar that there are many out there who replaced that trigger as soon as they cut the tape on the box.

As for the actual shooting experience – I have no complaints. It shoots like a SIG should. It took the boys at SIG a long while to release the MPX. There were problems and setbacks, but they got through them and released the MPX-P sometime after the standard one came out.

MPX Mags

The MPX magazine (manufactured exclusively for SIG by Lancer) is currently designated only for the MPX. There are Gen I mags and Gen II mags, corresponding with Gen I and II of the MPX itself. These magazines are expensive, and right now they are only available through SIG. The magazines are polymer, and they come in 10-, 20- and 30-round options.

There are some conflicting stories regarding compatibility or lack of it, but Gen II magazines should work on Gen I MPX guns. They did for me, anyway, when I was at the range. If your experience was different, or if you want to be sure of what you should do, simply get in touch with SIG. Better safe than sorry. Their reps know more than I do, that’s for sure.

Safety and Sights

The safety, as previously mentioned, is like a standard AR fire selector. It is ambidextrous and proves to be a bit too big for some shooters. It really does depend on the size of your hands, and on what you are used to shooting.

Firing a big gun, one that is actually meant to be a submachine gun can be a little awkward at first – especially with no stock to speak of. At any rate, the safety works fine, and its inner mechanism makes it one of the safest SMGs out there.

Sig Sauer MPX PSB

The sights on the MPX are factory iron flip-ups, but the Picatinny rail means that those will probably serve as backup sights (highly recommended) and that a more reliable optic will be mounted onto the weapon. Take your pick of red dot, laser, flashlight, and get to it.

I mean, sure, you could stick with the flip-ups, but that Picatinny rail is there for a reason, so you may as well use it. Mounting a red dot or other optic is the best way to take advantage of this firearm, and it allows you to remain armed and ready in the day or night, and increase the gun’s range and accuracy.


Accessories vary from one version to the next. The different models allow the user to change the stock configuration, barrel length, and calibers, and also make use of braces, suppressors, muzzle brakes, flash hiders, and other attachments.

Part of what makes this such a great gun is that these adjustments and alterations can be done in the field, on the fly. This is a highly adaptable weapon, and it has the ability to change according to mission specifications.

SIG Sauer MPX For Civilian Use

The original MPX came fully automatic, so when the question of civilian use came into the picture, the semi-automatic versions were obviously the way to go. Civilians have their choice of the SIG MPX Carbine, Carbine FDE (flat dark earth), Pistol, PSB (pistol stabilizing brace), PSB FDE, and SBR (short-barreled rifle) models.

SIG Sauer MPX SBR Variant

The SBR variant is another popular choice. Gun laws in the U.S. are weird sometimes, no doubt about it. In most cases, possession of an SBR requires a special ATF tax stamp. Without it, you could get in trouble with federal authorities.

The MPX SBR comes in a 4.5-inch barrel or 8-inch barrel configuration. The gun comes with a single 30-round polymer magazine, folding sights, collapsible stock, polymer grip, MIL-SPEC trigger, carbon steel barrel, and hard coat anodized receiver.


  • 4.5″ Barrel: $1957
  • 8″ Barrel: $2123

MPX Copperhead

SIG unveiled the newest addition to their popular MPX lineup at the 2019 SHOT Show: the Copperhead model. Like others of the MPX line, it’s somewhat unorthodox in design and appearance, and it certainly deserves a mention.

Some history: In 2018, the US Army requested manufacturers to submit a subcompact weapon prototype – one which was 9mm, Picatinny rail-topped, selective fire, and sound-suppressed. SIG submitted the MPX, and this is the civilian version of that weapon.

Some specs:

  • Caliber: 9mm
  • Length: 14.5″
  • Barrel length: 3.5″
  • Weight: 4.5 lbs
  • Capcity: 10, 20, or 30 rounds (MPX mag)
  • MSRP: $1,835

It is gas-piston operated, with controls which highly resemble an AR-style weapon, and a darn-tan copper-ish color. It features a different pistol grip than previous versions, dual recoil springs with metal retainer, a dual-position telescopic brace, one-piece upper receiver, along with an integrated muzzle brake and flash hider.

MPX Copperhead
Sig Sauer MPX Copperhead

To quote a gun-forum visitor: “It’s big, heavy, and ugly. I’ll take two.”

MPX Availability

The MSRP of the various MPX models ranges from $1852 – $2152. These are the starting prices, as listed on SIG’s website. As always, the actual street price will probably be lower.

This is a gun which will not fit everyone’s budget. Some may even go so far as to call this a luxury gun. It could be, sure. But that is true to all SIG firearms, to some extent. They cost more, and they always have. No getting around that.

This is a piece of hardware that is meant to save the lives of you and your loved ones. If the price is too high, there are always more affordable options. Alternatively, you could wait for a significant sale to cross your path. Those happen a lot, so keep your eyes and ears open.

MPX variants are available online and offline, in gun shops and firearms shows or conventions. SIG also work with specifically authorized dealers, so if you want to find the closest one to you, use their website’s resources! You will undoubtedly be able to find a good deal on one, somewhere.


The SIG MPX submachine gun, whether in fully-automatic or semi-automatic configuration, is a gun which has been many years in the making. It is not the very first submachine gun to utilize a gas system, but it is definitely the first one to pull it off so beautifully and flawlessly. And in 9X19mm Parabellum, too!

Overall, it is a great weapon. I am glad I had the chance to try it out in its pistol configuration, but let’s be honest – it is the full auto ‘beast’ configuration that seems to be the most appealing, at least range-wise. Shooting anything in full auto mode is a lot of fun, but it eats up ammo like there’s no tomorrow.

As far as I am concerned, the only true purpose of an individual with a fully automatic weapon is providing proper cover fire. Remember, this is a pistol. A big one, yes, but a pistol nonetheless. And like a pistol, it is meant for relatively short ranges. It is not a battle rifle or machine gun, it is a gun.

So, if you want it – and can afford it – go for it! The MPX is guaranteed to be one hell of an addition to your arsenal. And if you have any extra cash, send one my way, too!

Sam M

Sam is an avid firearms enthusiast who loves sharing his knowledge and experience with fellow gunivores.

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