SIG SAUER MPX Copperhead Bites with SMG Potency A Revival of the SMG


The trend towards shrinking firearms into smaller and lighter packages is full steam ahead. The recently introduced SIG SAUER MPX Copperhead is a perfect example of this. The MPX Copperhead is a derivative within their MPX 9mm SMG product line. Similar to other weapons within the SIG SAUER catalogue, the MPX Copperhead was a response to a specific customer request. The MPX Copperhead was purposely built to be the smallest possible size for carry in a bag or in a vehicle weapon. This is reflected by the 3.5-inch barrel, allowing for an overall length of 14.5 inches! Its weight is a feathery 4.5 pounds. This is not a misprint.

Some would have you believe we have arrived at the end of the submachine gun (SMG) as a viable weapon class. They point to the SMG’s steady relegation to the sidelines in terms of use since the end of WWII. The first impetus of this path occurred with the rise of the assault rifle, characterized by controllable select-fire capability with its intermediary round compared to the full-powered main battle rifles of the early 20th century. The appearance of Personal Defense Weapons (PDWs) seemed the final death knell of the SMG.

The PDW is characterized by its compact size, much like an SMG—centered on new cartridge designs facilitating the penetration of body armor out to a couple hundred yards. The PDWs search was stimulated when NATO had images of body-armored Spetsnaz troops running amok in rear echelon areas. The FN 5.7 and HK 4.6 are two of the more prominent examples of PDW cartridge types. Times and threats have changed, breathing new life back into the SMG genre. While certainly not comparable to a rifle cartridge, a 115- to 147- grain 9mm instills more confidence than a PDW’s 23-grain .22 caliber bullet (or smaller) when having to put down an opponent in close quarters.

Recent events illustrate what a dangerous world we inhabit with terrorist attacks growing in frequency. If a hostile situation is encountered, the civilian will most likely be defending himself with a handgun. A handgun is definitely not the optimum choice, especially when compared to a rifle. A handgun is a compromise between portability and performance. A compact concealed carry handgun is an even further compromise along this continuum, i.e., not your preferred full-size high-capacity blaster. A weapon like the braced SIG MPX Copperhead, which legally speaking is a handgun, concealed in a small discreet pack could very well be considered a force multiplier compared to a prototypical handgun.

The SMG’s most prominent role, indeed its raison d’être stretching back to the WWI trenches, is as a CQB weapon. The increase in terrorism, organized crime and drug smuggling with the resultant militarization of law enforcement teams to deal with these well-armed opponents has created a new need for the SMG type of weapon. Along these same lines civilians concerned and willing to take more responsibility for their own security can find a solution to their conundrum with a weapon like the MPX Copperhead. The SIG SAUER MPX Copperhead is poised to reassert SMG potency and viability.

The initial SIG SAUER MPX model was a much-heralded introduction at the 2013 SHOT Show. SIG’s goal with the MPX was to provide an updated replacement for aging submachine guns found worldwide. The MPX class, including the Copperhead, operates via a scaled-down, short-stroke gas piston adapted for use with pistol caliber cartridges. The use of a fully closed and locked rotating bolt is another enhancement. This sets the MPX apart from previous SMG designs which operated via blowback, recoil or roller-delayed methods. The SIG short-stroke gas piston allows the SIG MPX SMG series of weapons to run all weights and brands of 9mm ammunition with no adjustments to the gas valve. MPX operating controls and basic ergonomics are similar to the ever so prevalent AR-15; an important consideration for training/orientation purposes. An AR-like ambidextrous magazine and bolt release along with a centrally located non-reciprocating charging handle will be instantly familiar to any AR user. An AR M4 flat-top upper allows for any AR-type sighting system. This AR-centric theme is reinforced by using an AR-type fire control system.


The distinguishing MPX Copperhead features are the 3.5-inch barrel with integrated brake, monolithic Elite Series Cerakote finish upper receiver with an integrated stock knuckle lower and new SIG SAUER Pivoting Contour Brace (PCB), meeting customer desire for more compactness. The PCB brace easily conforms to the movement of the shooter’s arm with a patented swivel operation for adaptable placement and can be rapidly deployed via extended backward from the receiver. Some may complain of the lack of a threaded barrel on the MPX Copperhead for mounting a suppressor. When queried about this, SIG answered if suppressed fire is a desired trait then another member of the MPX family can be chosen. The MPX Copperhead’s integrated brake allows for the shortest barrel configuration possible. The Copperhead’s design intent was compact firepower … period.

No weapon, no matter what theoretical advantage is offered, is worth considering if not reliable. As with most automatics, magazines are a crucial linchpin. Reports indicate SIG SAUER worked with Lancer to develop the polymer magazines for the MPX. Lancer took advantage of their experience with AR-15 magazines to create a translucent magazine with steel reinforcing inserts at the feed lips and a magazine catch slot. The MPX magazines are designed to present the next round almost directly in line with the chamber. This enhances reliability with the full-auto MPX versions’ high cyclic rate. Another benefit of this is allowing for the use of non-FMJ profile bullets; i.e., JHPs—something important for law enforcement or personal defense considerations. One note here is that one must pay attention when purchasing MPX magazines. Make sure Gen 2 MPX magazines are used with the MPX Copperhead. Gen 1 magazines that were tested proved problematic.

The SIG SAUER MPX Copperhead’s non-discriminatory handling of several types of 9mm ammunition was a pleasant surprise. Winchester 9mm NATO loads were initially tested based on experience with older 9mm SMG designs requiring the higher-pressure NATO loads to operate reliably. The MPX Copperhead functioned flawlessly, not only with the Winchester 9mm 124-grain NATO FMJ loads, but also with SIG SAUER 365 FMJ/Elite and V-Crown JHP. Federal’s new Syntech loads were also employed. The ability to feed the JHP and other bullet profiles was a totally unexpected bonus and a credit to the MPX design and further increases its potency as a CQB weapon for LE and civilians.

Find What Works

We might as well speak about the gorilla in the room when it comes to weapons turned into pistols from designs more associated with stocked SMG or rifles. What does a pistol like SIG SAUER MPX Copperhead offer? The answer here is nebulous based on user expectations and intentions. For many, a non-traditional pistol like the MPX Copperhead, while enticing due to firepower potential, lacks much in terms of accuracy past normal handgun ranges with an added negative of weighing twice as much as a typical handgun. Another possible method could be derived from how certain European special operation units run their weapons. The end of the weapon is floated on the cheek providing three points of contact for more stability and thus accuracy. While not optimum and not nearly as effective as a true shouldered stock, the alternate methods proved superior to merely attempting to hold the MPX Copperhead via traditional pistol method, especially when the goal is rapid fire rounds downrange with a semblance of accuracy. Attentive training will pay dividends in finding what works best for individual users.


The SIG SAUER MPX Copperhead was tested at Echo Valley Training Center. A SIG SAUER ROMEO 4 red dot was mounted. Iron sights are not provided with the MPX Copperhead. Several 30-round magazines were fired rapidly downrange. The MPX Copperhead has an ambidextrous charging handle that is very similar in appearance and function as the AR-15. It was determined that manual operation worked best by placing fingers on both sides of the charging handle so as to pull back evenly. The ambidextrous safety and magazine levers were found in familiar places to anyone who has operated an AR. The MPX Copperhead proved durable during testing at EVTC with nearly 700 rounds fired over multiple range visits. Most importantly the SIG SAUER MPX Copperhead demonstrated reliability. EVTC’s Hesco Shoot House was a popular venue for testing.

Magazine changes with the MPX Copperhead proved a smooth affair indicative of AR-like ergonomics. The closed-bolt MPX does have a bolt hold-open device which is different than most SMG designs, aiding in swifter reloading. The balance of the MPX Copperhead combined with firing 9mm in a 4.5-pound weapon minimizes any recoil or muzzle climb. This is especially important in the MPX Copperhead’s select-fire brethren with a cyclic rate over 1,000 rounds per minute (rpm). The piston-operated rotating bolt MPX contributes here as well. Bolt weight is minimal compared to more traditional SMG blowback methods featuring a hefty chunk of metal for operation. Strings of multiple rounds sent downrange smacked steel with no problem due to minimal muzzle climb. The diminutive nature of the MPX Copperhead can cause nervousness in users based on hand placement on the pint-size weapon. You definitely do not want to take a high thumb forward grip.

When everything is stripped away in terms of its design innovations, aesthetics and ergonomics, at its core the SIG SAUER MPX Copperhead is a compact 9mm SMG intended to put rounds on target in a hostile environment. The MPX Copperhead is lighter and handles better than its full-powered assault rifle brethren. While not as prevalent of late, there is an economic advantage in terms of the 9mm amount that can be purchased versus a rifle cartridge. These economics can be taken advantage of with more training; not to mention the ability to access indoor ranges or shoot houses with a pistol caliber weapon such as the MPX Copperhead. The training regimen is further enhanced by using 9mm with steel plate targets without having to resort to specialty frangible rifle loads. The Federal Syntech loads are especially suited for engaging steel at close range with minimal chance of bullet spalling/splashback. Also, less experienced shooters will find the lower recoil pulse and muzzle blast of a pistol caliber easier to manage, which often times will translate into better accuracy potential.

The SIG SAUER MPX Copperhead represents the revival of the SMG. The MPX Copperhead will force a reevaluation of SMG use for law enforcement or direct-action military units. Most engagements are successful within a 100-yard range, especially in LE, MOUT or civilian settings with handling and reliability playing more of a factor in quick, reactive engagements versus the specific caliber used. Along these same lines a civilian concerned with home defense will find the MPX Copperhead hard to resist with its high capacity and great handling. The muzzle blast differential between a 9mm versus an obnoxious rifle muzzle signature is further accentuated in confined spaces—such as inside a home. Will a rifle cartridge hit with more authority than a 9mm? Yes, of course, but mission specifics may dictate when the SIG SAUER MPX Copperhead is a more practical choice of weaponry.



Federal Ammunition

Echo Valley Training Center

Hornady Mfg. Co.