The MPX family of pistol-caliber ﬁrearms ﬁxes the main ﬂaw of close-bolt blowback designs: excessive bolt weight. Adapting the AR-15 platform to 9×19 Luger with a gas-piston action, SIG engineers cut the overall weight and the reciprocating bolt carrier in particular, making MPX lighter than other 9mm ARs and cutting the recoil intensity at the same time. The resulting weapon is available as a 16-inch carbine, and as submachine gun, short barrel riﬂe and pistol, all available with 8-inch or 4.5-inch barrels.
In the carbine form, the 7.6-pound overall weight of the weapon is no different from a riﬂe-caliber AR-15, making it more of a practice version of the 5.56, with less expensive ammo, less concussive report but substantially similar handling and manual of arms. The shorter barrel and forend of the 8-inch SBR and submachine gun variants bring the weight down to 6 pounds, and collapsed length down to 17 inches.
Unfortunately, National Firearms Act restrictions make the SMG unavailable except to government or corporate users, and the tax stamp and yearlong ATF turnaround on approving applications restrict the SBR. That leaves the pistol as the less legally encumbered purchase that can be turned into an SBR at a later date.
The 9mm Luger cartridge generated far smaller volume of gas than 5.56x45mm, so the MPX gas port is almost right at the chamber to generate sufficient pressure for cycling. With most 9mm loads, 8 inches is sufficient to get most of the potential velocity increase from the limited case volume. With the A2 ﬂash hider, the muzzle signature is nonexistent.
As with other gas-operated pistol-caliber guns, the MPX favors full-power ammunition for reliability – in my testing, it ran perfectly with 115-, 124- and 147-grain SIGbrand defense and range ammunition, but short-stroked occasionally with wimpy commercial remanufactured ball. With full-power ammunition, MPX has less felt recoil than blowback guns had with subpar loads.
WHEN SUPPORTED, the MPX pistol is superbly accurate. When rested on an convenient cardboard box and sighted with a red dot, the pistol shot very small groups at 25 yards, especially favoring 124- and 147-grain SIG JHP ammunition.
Similar or slightly better results were obtained using the MPX submachine gun in semiautomatic mode. In auto mode, running at about 850 rounds per minute, it remains fairly controllable and will keep two- or three-shot bursts in A zone at 25 yards. The mechanics of the MPX design are very sound. Compared to HK MP5, it runs a good deal cleaner, especially when sound-suppressed. Takedown for cleaning and especially the reassembly are much simpler, with all bolt and carrier parts accessible with the removal of a single pin.
MPX ergonomics are similar to AR-15, but with an emphasis on ambidextrous controls. Slide lock levers and magazine release buttons are duplicated on both sides, a helpful feature. On the left side, the controls could use more separation, as trying to lock the slide back sometimes caused a dropped magazine. The transparent, metal-reinforced polymer magazines made by Lancer are extremely reliable, durable and were easy to load. While more expensive than typically used single-feed Glock magazines, they are far more convenient in use. Available in 10-, 20- and 30-round capacity, MPX magazines ﬁt any purpose, from combat to concealed carry to shooting from a range bench.
THE PRINCIPAL DIFFERENCE between the SBR and the pistol is ergonomics. The pistol comes with a QD socket at the rear of the receiver, right under the rail for the arm brace or the stock. In theory, a solid shooting position can be established with the use of both hands and a stretched sling. In practice, holding a 6-pound weapon in outstretched arms gets tiring fairly soon. Practical accuracy is no better than with a conventional pistol, and the sling length and position make effective concealment difficult.
Furthermore, the ambidextrous charging handle retained from the AR-15 has a tendency to entangle with the plastic sling ﬁxtures, pulling the bolt out of battery and disabling the gun. At close range, especially indoors, the MPX pistol would be more stable if ﬁred from the hip using a green laser for aiming.
In my opinion, the best ﬁghting pistol made by SIG would be something like a full-size P226. The MPX is terriﬁc as a carbine or a submachine gun, but – thanks to ﬁlling a regulatory niche created by illogical government regulations – is a pistol in name only. In reality, it’s a stockless carbine and would be best treated as a pre-SBR that the owner gets to take home before the tax stamp arrives.
If NFA regulations and restrictions aren’t your cup of tea, the 16-inch version of the MPX is superbly accurate, has almost no felt recoil and has a proper stock without requiring a tax stamp. For unsuppressed use, carbine-speciﬁc 9mm loads, such as 77- (2,000 feet per second) or 115-grain (1,500 fps) Overwatch, provide ﬂat trajectory and effective terminal ballistics. From the 8-inch barrel, Sig V-Crown defensive loads are superior. With lower muzzle pressure than the pistol it also suppressed even more effectively, particularly with the SIG subsonic 147-grain load.
Unlike the 5.56mm AR-15, the MPX has no perceptible gas blowback reaching the shooter. Given the excellence of the MPX concept, we can only hope that NFA regulations would be rolled back in the coming year, putting all of its features into the hands of a large and very appreciative group of American ﬁrearms enthusiasts. ASJ
Editor’s note: For more on SIG Sauer’s MPX line, see sigsauer.com.