The Sig Sauer MPX

Over the past two decades, the 9 mm SMG has all but died in the law enforcement and military roles in favor of the 5.56 mm patrol carbine, CQB and SWAT weapons. Until this point the Heckler & Koch MP5 was the sole ruling weapon. Sure there were a few others like the UZI and Colt SMG, but the MP5 was King. The demise of the 9×19 mm weapon was really two-fold; first were several LE encounters with body armor where the 9×19 mm MP5’s left the officers defenseless, and second was primarily education showing that the rifle powered 5.56 mm rounds had less penetration in urban environments than the 9×19 mm. Projectiles from the 5.56 mm would break apart in wall board and wood where the slower and heavier 9×19 mm would go right through. Another trend was that the trusty 12-gauge shotgun, which had been in squad cars for nearly a century, were being replaced with 5.56 mm patrol rifles. Due to active shooting crime scenes, if the suspect had body armor the patrol officer with his pistol and shotgun could not stop him. They would have to wait until SWAT to arrived. Now with giving them a rifle, they will be able to engage and stop the target on the spot without having to wait for SWAT arrive. Also there is the question of collateral damage. It is easier to account for 1 round than it is for 9 uncontrolled OO Buck pellets. Though this is the trend, it is an American trend. The rest of the world for the most part still likes the concept of the sub machinegun and accept it as a viable military and law enforcement weapon.

In the US, there has been little SMG development. Perhaps Quarter Circle 10 has done the most in this area in recent times. However, at the time SIG Sauer in the US was working on a requirement-driven program for a suppressed 300 Blackout rifle called the MCX. The MCX is quite modular in nature. The upper receiver is a single 6061 T6 aluminum extrusion with a removable handguard. The handguard is locked in place by the front pivot pin and comes in different lengths (short and long) which are determined by the barrel length desired. The barrel itself is held in place by a set screw clamping mechanism and allows with the turn of two bolts to remove the barrel assembly. The barrel assembly consists of the barrel, gas block, gas valve and short stroke piston as well as flash suppressor/suppressor mount. Although developed for the .300 Blackout cartridge it was soon adapted to a 5.56 mm barrel as well. The bolt is a standard AR15 bolt with a half length bolt carrier. The cam track is the same as any AR15. No buffer or buffer springs are used. Instead dual action springs are mounted to the bolt carrier group. This enables the rifle to function with or without a stock. The action springs are inside the receiver.

Development of the new Sig 9mm MPX began in 2012 around the same time as the MCX. SIG has released several models of the MPX, but engineering and development continue to widen the capability and diversity of the MPX system. According to SIG, the MPX was developed as an individual program based on the needs of various military customers. As time went on, both the MCX and MPX began to merge together as a sort of big brother/ little brother family of weapons. SIG felt that they could apply this to a pistol caliber as well. So they did and the MPX was born. The MPX is very similar in features to that of the MCX.

The lower receiver is made from a 7075 T6 aluminum forging. It takes a standard AR-type trigger group; however is not recommended due to the higher impulse of the gun. The trigger group is designed as a “trigger bridge” that protects the disconnector from the hammer striking it and reduces trigger slap, making the shooting experience more enjoyable to the end user. SIG claims using other trigger groups will not void their warranty, but SIG will not replace after market trigger group components. The factory trigger broke right at 8.5 pounds. The selector lever and magazine release are ambidextrous. The ambidextrous bolt catch is quite convenient and easy to operate. There are five different stock variations which attach to the lower receiver via Mil-Std-1913 on the rear of the receiver. The first and most common is a telescopic stock similar to that of an MP5. There is a button on the top of the rear of the stock, and it has three positions. The second is a side folding stock that folds to the left. This is standard on the MCX. The third is a folding M4-type receiver extension which will accept any AR15/M16/M4 type stock. The fourth is a telescopic side folding stock. The fifth is a side folding pistol brace. The magazine well is not a converted 5.56 mm magazine well but designed specifically for their new magazine. There are gripping grooves on the front of the magazine as well.

The magazine was redesigned from the start. The standard Colt-type/UZI 9 mm magazine is durable and reliable. However it has some issues. First is it is extremely difficult to load. Especially when you get down to the last 10 or so rounds. To load comfortably you would need to use one of the MagLula loading tools. Second is if you drop the magazine it may very well unload itself. The new magazine was co-developed with Lancer Systems. It is manufactured from translucent polymer with steel feed lips. The magazine has a curved profile also similar to that of the MP5. The magazine is easy and comfortable to load. The magazine comes standard in 30 round capacity but is also offered in 20-round capacity variation as well. Although the magazine is manufactured by Lancer, it can only be purchased through SIG. This magazine was drop tested loaded, and the author did not experience any issues with the magazine breaking or cartridges flying out when dropped. Lancer is known for making some of the finest and most proven magazines in the industry. They know what mil-spec testing is and have had years of experience making some of the most durable magazines in the world.

Close up view of the right side of the receivers of the MPX. Notice the ambidextrous safety and bolt catch. This carbine is also equipped with a AR15-type ejection port dust cover. Also notice the translucent magazine manufactured by Lancer Systems.

The upper receiver, like the MCX is manufactured from a single extrusion of 6061 T6 aircraft grade aluminum forging. The ejection port is the appropriate size to function with pistol calibers 9×19 mm, .40 and .357 SIG-caliber ammunition. The receiver features a polymer ejection port dust cover similar to that of the AR15. The top rail is consistent Mil-Std-1913 rail. The removable rail system is set up as a Keymod rail which allows the end user to install rail segments of various lengths where they are needed. There are both a short-and-long-length handguard which is designed for use with the short 8-inch barrel, and the longer one is used with the 16-inch barrel. There are also suppressors that will fit inside the handguard. On top of the rail are folding back up iron front and rear sights. Looking inside the receiver, the first thing that pops out is the fixed ejector. This is always a reliability improvement over the spring loaded plungers. The ejector cannot get stuck in the bolt or have an ejector spring wear out or a retainer pin brake.

The charging handle is quite unique as well. It is “T”-shaped like a standard M16/M4-type but is not compatible. The charging handle is fully ambidextrous with latches on both the right and left side. It is quite comfortable and easy to use.

The provided test and evaluation MPX was equipped with an 8-inch barrel. The muzzle device was a birdcage-style flash suppressor. The barrel is nitride (FNC) finish and has six lands and grooves, right one turn in 10-inch twist. Barrel lengths are available from 8 and 16 inches in length. According to SIG, the customers may expect 4.5 and 6.5 inch barrels in the second quarter of 2016. The barrels are manufactured in 9×19 mm, .40 and .357 SIG but as of this writing, the .40 and .357 SIG barrels are not yet in production. The barrel assembly has the barrel, gas block, short stroke piston assembly and barrel extension. It will also have a flash suppressor mounted to the barrel.

One of the truly unique features of the MPX is that the barrel is easily removed from the upper receiver and changed out. You can change out barrel lengths! As previously stated the MPX is offered in 9×19 mm, .357 SIG and .40 caliber using the exact same upper receiver. So you will be able to swap calibers as well. You only need to change the barrel, bolt (.357 and .40 are same bolt) and magazine (.357 and .40 cal same mag). This author’s initial concern is that the pressures of the .357 and .40 caliber rounds are greater than the 9 mm and how the MPX would hold up. According to SIG, they claim they have reinforced the receivers where needed, and the recoil system is stout enough to handle the extra push. On the same note, I asked the question if the MPX is +P and +P+ rated. They said “absolutely. Keep in mind these are hot loads, and parts life should be expected to be lower if this is the primary ammunition.”

The bolt carrier group is very similar to the MCX. Unlike most 9mm-type carbines and SMGs, the MPX uses a short stroke tappet piston rotating bolt locking mechanism rather than a blowback operated mechanism. According to SIG, they feel safety is their #1 priority. They felt a rotating bolt provides better safety compared to the traditional blow back mechanism. If over pressurization happens within the chamber or bore of the firearm the operator would be much safer. Looking at the assembly, the bolt carrier has dual recoil springs. The bolt rotates by use of the same cam pin design of the standard M16/M4. There is a metal insert in the receiver to protect the aluminum from coming in contact with the head of the cam pin during the unlocking/recoiling phase of the cycle of operations. Normally one will see some gouging in the aluminum right behind the cam notch inside the upper receiver. This steel insert prevents this. There is no firing spring used like in the standard M16-type 9 mm carbine or SMG. Due to the rotating bolt it is not necessary like on the blowback-operated mechanism. The bolt carrier comes apart identical to the standard M16 utilizing a firing pin retainer pin, firing pin, cam pin, extractor pin, extractor and bolt.

Notice the ambidextrous charging handle. The stock has two positions; The button located behind the charging handle is depressed to unlock the telescopic stock assembly.

As of this writing the MPX is offered in five configurations. First is the SIG MPX carbine (MPX-C-9KM-T) which has a 16 inch barrel with long handguard. This variation comes with the telescopic stock, back up sights and one 30 round magazine. It weighs 7.6 pounds with an overall length of 28.5 inches with stock fully collapsed and 33 inches extended. The next is the SIG MPX SBR (MPX-9-T-KM-SBR). This version has an 8-inch barrel with short handguard. This model weighs 6 pounds with an overall length of 21 inches with stock fully collapsed and 25.25 inches extended. The SIG MPX-K SBR (MPX-K-9-T-KM-SBR) has a 4.5 inch barrel and weighs just 5.7 pounds. It also comes with a three position telescopic stock with an overall length of 17.5 inches with stock fully collapsed and 22 inches extended. The SIG PMPX with PSD is a SIG MPX with the side folding stock with an 8-inch barrel. It weighs 6 pounds with an overall length of 17.5 inches with stock folded and 25.75 inches extended. The final is the pistol configuration SIG PMPX (MPX-P-9-KM). This model has no stock or may be had with a SIG Brace. The SIG brace is still under scrutiny of BATF. If used as designed it is legal but if one puts the rear of the SIG Brace on his shoulder it now becomes a Short Barreled Rifle and subject to NFA laws. This controversy is far beyond the scope of the review of the MPX. This pistol version has 8 inch barrel with a weight of 5 pounds and an overall length of 17 inches. The SIG MPX is 100% designed and manufactured in the USA at their New Hampshire facility.

For an optic, the Aimpoint Micro T-1 2MOA red dot site was chosen. This is a compact version of the military combat proven Comp4. With the mount with which it was supplied there was complete co-witness with the iron sights due to the Micro Spacer High mount. The sight is powered by a CR2032 battery. The intensity of the dot has 12 positions. The optic has a 1x magnification. The aiming dot size can be 2 or 4 MOA. The battery life is about 5 years if left on continuously in setting 8. The sight weighs only 3.7 ounces with a length of 2 7/16 inches and a width of 1.9/16 inches. The sight is manufactured from high strength aluminum. The sight has 4four night vision compatible settings.

Around the same time the MPX arrived, so did some of the new SIG Elite Performance Ammunition. SIG has announced and began shipping a whole line of target and high performance hollow point ammunition in an array of pistol calibers. The SIG V-Crown™ JHP is offered in .380 Auto, 9 mm Luger, .357 SIG, .38 Super +P, .38 Special +P, .357 Magnum, .40 S&W, 10 mm, .45 Auto, .44 S&W Special, .44 Magnum and .45 Long Colt. This is a high performance hollow point . The 9 mm Luger tested was loaded with a .124gr projectile. When the projectile expands it has a similar look to the older PMC Starfire projectile. The cartridge case is nickel plated for increased reliable feeding. The velocity is 1189 feet per second on this cartridge. Sigs target line is called SIG FMJ which are currently available in 380 Auto, 9 mm Luger, .357 SIG, .38 Super +P, .38 Special +P, .357 Magnum, .40 S&W, 10 mm (full powered load) and .45 Auto. Received was 100 rounds of the 9 mm SIG full metal jacket which is a .115 gr full metal jacket round nose with a muzzle velocity of 1185 feet per second. The brass has a SIG headstamp on it. The ammunition is clean and spotless. I received another 100 rounds of this ammunition. Additionally 300 rounds of Black Hills Ammunition 9 mm 115gr full metal jacket ammunition.

The Sig Sauer MPX field stripped for cleaning. The Sig MCX and MPX come apart identically. Notice how the stock is attached by Mil-Std-1913 rail. No tools needed for disassembly.

In all 500 rounds fired, there were no malfunctions of any type. There was only one magazine provided with this MPX. It was found it could be difficult to load a full 30 rounds in the magazine and insert it on a closed bolt. Similar to that of a standard 30 round GI aluminum M16 magazine which also shares this issue. By downloading 1 round this problem went away. Recoil was moderate. This author is well known to be an M16/M4 fan. The feel of this was familiar and easy to use. Every control was where you are used to it being. The bolt catch was a nice addition. The telescopic stock was nice and perhaps comfortable to a normal size person. The stock was a little on the short side. Unfortunately, the other calibers were not available to test. Testing was done at a 25 yard range. All ammunition performed equally well. All groups were just over an inch. This MPX did not seem to favor one ammo over another. Perhaps if longer range was tested it may have made some difference. The firearm is what one would expect to see from SIG. Well thought out, well designed and nicely manufactured. Like all SIG firearms, they are not cheap! The MSRP on the MPX family of weapons is from $1,500 to $1,960. But you will get what you pay for. With the increased use of sound suppressors, SIG has designed and manufactured their own suppressors for this weapon system. Also with the increase in sales of short barreled rifles, the SBR models are very popular. Of all of the 9 mm carbines out there, the SIG is without a doubt the most advanced. With the quick-change barrel and gas-operated rotating bolt mechanism, this is a modular, possible over-built for safety and multi caliber new generation SMG/Carbine.