Revolutionizing Defense

2A Gun Writer Pistols, Red Dot 1 Comment

Considered a Pistol, the MPX folding brace does not impede the end user while shooting

MP5, MP40, Thompson M1921 and even the “frankengun” Luger P08. All sub guns, who helped solidify the debate of whether a pistol caliber shoulder-fired weapons’ effectiveness at various ranges. Some more aesthetically pleasing than others; while shooting in close quarters, they all hold tons of value. Sig stepped up their game and introduced their line of sub guns. Coined the MPX. Due to its versatility in rapid barrel changes, immediate adaptability between platforms. Similar sub guns all but have the end user pigeon holed into one end use. The “X” has a variance of either “pistol”, or short barreled rifle (SBR). The moving parts remain the same between both, however; as many are aware…the difference between the two is earmarked by a brace that closely resembles a buttstock or no brace at all and a full telescoping stock. If you have a SOT or live in an SBR friendly state; Sig sells the MPX as an SBR which comes with a collapsible/telescoping buttstock.


Whether in the Marines or on the private side of the house, HK MP5’s and Colt SMG’s were always fun to work with, but completely unattainable and impractical for stateside defensive or “shooter” purposes both on the availability and price point. Once Sig released the MPX, in our immediate opinion it is more aesthetically pleasing than the MP5 and beats the brakes off the AR look alike SMG commonly used by federal agencies. In our instance, its folding brace attaches directly to the back of the gun, secured into place by a Torx screw.

The Pistol Stabilization Brace (PSB) is a quick solution for shouldering options

The MPX is completely ambidextrous. A feature that our left handed 10-year-old model truly appreciated.  Beginning with Sigs ambi charging handle, loading and clearing are about as effortless as you can get with a gun this small. The MPX operates from a unique short stroke gas piston system. This piston system is all internal to the upper receiver. An “ambi” charging handle is crucial in smooth operation, significantly reducing shooter induced malfunctions every time it is manipulated.

The MPX is completely ambidextrous

Ambi mag releases and bolt latches are underappreciated functions across many weapon platforms. The number of times we have had to transition to support handed shooting are countless due to cover restraints, ergonomics or both.  Muscle memory kicked in and we found our hands virtually tied in knots trying to reload or clear stoppages while shouldered in our support side. Yes, we created a training scar by simply not training these scenarios at all…the MPX promotes seamless training for those forward thinkers. The MPX was to control throughout our testing due to the ergonomics. From magazine changes to selector manipulation, one side mirrors the other for repetitious operation. If we had our druthers, they would be to install a bolt release on the strong (right) side of the MPX. Doing so would allow mirrored operation in its entirety, and could promote ambidextrous firearms manipulation in training methods and theory.

The objective Sig achieve with the  MPX was to mirror the AR platform as closely as they could, simply because it [the AR] is one of the most popular systems on the planet, virtually everyone who shoots is familiar with them. In true “Sig fashion, ” they stuck with what work and made necessary modifications to areas where they saw failures. Their approach to the MPX design was practicality, functionality, and interoperability no matter what the hard end use be.

Aside from the unique operating system design, everything about the MPX mirrors the tried and true AR – 15 platforms. We understand that there are some that hate it [AR-15] but there is something to be said for the platform. Sig mirrored the rotating locking bolt design that ensures consistent firing. The carrier and bolt closely resemble that of the “black gun”, just smaller. The bolt maintains its’ ¼ turn travel locking it into the chamber before the pin strikes the primer.

The MPX unique piston and operating rod/spring system accounts for its flawless operation

The MPX fire control group is compatible with the 2 pin AR 15 Triggers

Squeezing off rounds is not a chore and does not come as a surprise either. It’s standard 2-pin trigger, breaks at a consistent 7lbs with very little creep on the front end and virtually no over travel on the back. IF you’re a trigger snob, The MPX allows for aftermarket trigger installation against Sigs recommendations. From our understanding, there are plenty of those who have done it without fail. Fair warning…if you do and you have a catastrophic failure, you could be out the money you invested in the gun by its warranty being void. We saw no immediate need to replace the trigger that came with the gun, it was adequate in our use. Keep in mind, if you’re not taking our word for it…the typical purpose for replacing triggers in shoulder guns is to eliminate as much human error in precision shots as possible…

The MPX is 9mm…you’re not taking 300 yard shots with it…The gun is meant for close quarter engagements; just saying!


The MPX PSB has 13 inches of prime real estate to mount any Picatinny compatible optic to

Range Time

We opted not to test iron sights on the MPX as we felt that the MRO would more than suffice for what we intended to use the MPX for. The monolithic upper receiver allows for quick installation of virtually any sight that you want; its top rail offers 13 inches of adequate distance between the shooters front and rear sights should you go that route.

We zeroed the MRO at 25 yards and our function test was underway. We quickly established a rhythm and felt comfortable with where the MPX trigger broke and reset. We ran through our standard controlled and hammered pair sequences of fire, then transition to failure drills and a “mag dump”. The modified A2 “bird cage” flash hider aided in mitigating un necessary muzzle flip during rapid fire iterations.

We understand and fully appreciate the lack of training value in the all too popular mag dumps, however doing so tests the functionality of the trigger group and all other moving parts of the gun.

Once our suspicions of optimal fire and function were confirmed, we began shooting at the 15 eventually ended up at 100-yard line. Its accuracy was spot on and everything you would expect out of a pistol caliber carbine. Naturally, from distances beyond 75 yards there was bullet drop, but knowing where to compensate for those distances is crucial to rounds on target.

Our testing for record maxed out at 50 yards as we do not want to blur the lines between practical and ridiculous.

We cleared the gun, gave a quick instructional overview of the gun and handed it off to our 10-year-old. The MPX length and weight were evenly matched and easy to manipulate. Without a sling, our model was transitioning steel targets effortlessly.  Its ambidextrous controls measured up with our shooters left handed manipulation. The “toe” of the brace pivots in the pocket of “J’s” shoulder, almost like he’s done it before. Our model has trained enough to know proper safety manipulation, so every time he presented the ambi selector would simultaneously be manipulated and controlled pairs would find themselves placed into the center of the Defense Target RST steel.  Even at ten years old, hammered pairs, failure drills, and multiple shots, multiple shot engagements were smooth and accurate, after all; the felt recoil of the MPX was virtually nonexistent, human error was the only thing that had to be contended with. The time between iterations decreased as our shooter adapted to the MRO as he adjusted the brightness.

Practical Application

Once we shot and tested the accuracy and function of the MPX, we carved out 30 days to carry the weapon from an off body carry point of view. The extent of our off-body carry was relegated to the range and stowing in our vehicle. We folded the stock, inserted a 30-round magazine and slid it into our Arcteryx Blade 28 backpack. Its small stature and compact design offer a level of concealability that similar guns (with stocks) do not.

Unzip the bag and establish a positive grip

Clear the pistol grip and brace from the Blade 28

Unfold the brace and scan for targets (if time permits)

Drop the Blade 28 and engage targets

Drawing and engaging targets from the Blade 28 took some practice, unzipping, removing the gun and drilling targets sounds easy…but until you do it…practice dry. We didn’t and ended up fumbling around for about an hour before we were engaging steel in under 5 seconds.

Finding a “permanent home” within a vehicle for a firearm like the MPX can be easy also vicarious. If you are one that travels a lot by vehicle, it can fit virtually anywhere. Between and under the seats specifically. Before storing the MPX in either place, know the laws specific to the transport of firearms in your state before you find a home for the MPX in your vehicle. Do yourself and our friends in blue a favor. If you are going to put the MPX under your seat, should you be pulled over, please let them know that you have it, and what condition you keep in. [loaded or unloaded]

Follow Through

In a seemingly endless sea of AR15, polymer framed pistols and “ultimate” defense weapons, the MPX has stood out to be one of the more versatile of the ones we have experience with. Sig has taken a giant step in the right direction for revolutionizing the sub gun or pistol caliber carbine. From the factory you will order and receive a specific model, the option remains if you would ever want to change something up. A Torx head screw makes the difference between a brace or K version of the MPX. The same screw can instantly turn your pistol into an SBR.

Our price to value assessment is also evenly balanced. Typically, anything with the modularity of the MPX, its price reflects. The MPX PSB (Pistol Stabilizing Brace) MSRP right above 2k. Yes, two grand is a lot of money for anything that you cannot drive, but what you get is Sig functioning with Sig versatility. The best part of it all, the MPX is readily available. We would recommend starting out with the MPX equipped with the Pistol Stabilizing Brace and customize from there. With any model of the MPX, your quest for the ultimate off body defense weapon could come to a screeching halt!


Sig Sauer MPX PSB Vital Statistics

Type Semi-Automatic
Gas Short Stroke Piston
Caliber 9mm Luger
Magazine Capacity 10, 20, 30 rounds
Handguard 8” one piece key mod
Barrel Length 8 Inches
Overall Length 27 ½ (extended) 17” Folded
Weight 6lbs 5oz (unloaded)
Stock Stabilization Brace
Grip Sig Polymer/Contoured
Trigger Pull 7lbs
Finish Type III hard coat anodized
Safety Selector Ambidextrous
Bolt/Mag Release Ambidextrous
MSRP $2,084.00
Manufacturer  Sig Sauer

Trijicon MRO SPECS

Type Reflex Red dot
Magnification 1x
Objective Lens 25mm
Length 2.6”
Weight 4.1oz w/out mount
Reticle color Red
Eye Relief Infinite
Adjustments ½ MOA
MSRP $579.00
Manufacturer Trijicon Optics

MPX PSB Ballistic Data

Data collected using Shooting Chrony Master chronograph, shooting three different loads

Hornady Critical Duty 135 gr

LO 999.5
HI 1019
Avg 1010
ES 19.69
SD 07.68


Hornady Custom XTP 124 gr

LO 1106
HI 1408
Avg 1184
ES 302.2
SD 126.7


Polycase ARX 74 gr

LO 1717
HI 1779
Avg 1754
ES 62.61
SD 22.91


MPX PSB Accuracy Results

Data collected by averaging four-5 round shot groups from 50 yards prone position, using Caldwell Stabilizing shooting bag and measured with Mitutoyo tuned calipers


Hornady Critical Duty 135 gr

Best Group 1.18”
Worst Group 1.50”
Average Group 1.02”

At just over an inch, the heavier Hornady round performed decently


Hornady Custom XTP 124 gr

Best Group .90”
Worst Group 1.26”
Average Group .86”

Hornady 124 Grain Custom XTP proved to reign supreme through the MPX spreading the calipers at .86″ average


Polycase ARX 74 gr

Best Group .98”
Worst Group 1.79”
Average Group 1.10”

The ARX defense rounds prove to be light, fast and are abundantly adequate in the purpose they were designed. We averaged just over 1 inch for accuracy.

About This Author

2A Gun Writer

Comments 1

  1. Gary,
    Thank you for reading and commenting. We take the total group spread and average them between the number of groups fired with the particular load.

Leave a Reply

Your email address will not be published. Required fields are marked *