Review: Sig MPX SBR

We put the Sig MPX SBR through its paces. How did it handle at the range?

Review: Sig MPX SBR

A decade after the U.S. pistol caliber carbine (PCC) market leapt forward, the PCC market remains one of the hottest-growing firearms segments thanks to the continued growth of SBRs and stabilizer brace options. Add in the rising cost of ammo and the reduced cost of shooting a PCC and the market has caught fire. With all the new AR PCC formats on the market, one of the biggest splashes in the market back in 2013 was the introduction of the 9mm Sig MPX format in pistol, rifle and SBR models. Oddly enough, Sig was not responding solely to the then-growing PCC market, but was offering it as an option to the H&K MP5 pistol and SBR prices, which were easily hitting the $3,500 mark for very used guns. The Sig MPX and other similarly designed models are still seeing accelerated growth because of the reliability, accuracy and reduced shooting cost of the platform over a 5.56-chambered option. The size, accuracy, recoil and capacity also have made it a popular home defense firearm. All these things pulled me into the MPX concept and gave me faith that my investment in the MPX and the extra $200 SBR tax stamp would bring grins at the range and not tears.

After owning my first braced 8-inch-barreled MPX, I felt more than comfortable investing in a Sig Factory SBR version of the format, which at the time was the only 4-inch-barreled model available. A few months after dropping the cash on the factory SBR, the 4-inch braced model became available. I purchased the backordered braced MPX and still had it in hand before the ATF SBR tax stamp arrived for my SBR purchase. Based on the 2A sentiment, I would do it again, and if required, SBR my other two MPXs. After shooting thousands of rounds through each of the three Sig MPX models I own, the reliability, accuracy and design has proven itself over the last seven years of my two factory Sig MPX braced pistols and a registered factory SBR model.

One of the biggest pitfalls and risks customers take on with expensive-to-purchase and hard-to-sell ATF-regulated SBRs is investing in a form factor which could ultimately prove unreliable, unsupported, and ultimately disappointing. The Sig MPX is a platform that is well proven, supported, and warranted, which customers might not get with other PCC SBRs. At the time of the purchase, I was dubious as to the longevity of the pistol brace concept and wanted an unquestionably legal PDW in a format I could put hard use into. During the pre-Trump years, there was a potential for a very aggressive 2A attack, and we could face the same thing over the next four years. It seemed to me then and now that owning ATF regulations firearms which are already meeting even the most extreme registration criteria seems like some degree of insurance to continue 2A rights.

Proven Reliability

I have owned a number of PCC formats, and the Sig MPX is the only option that has been reliable out of the box, with the exception of a factory-tuned Primary Weapons Systems PCC. PCCs can be problematic and extremely finicky, primarily due to the wide variety of 9mm pressures and resulting recoil, but the MPX platform’s unique gas piston design delivers perfect reliability, with hyper-velocity Liberty 2,040 fps 50-grain rounds all the way down to 158-grain subsonic 850 fps Fiocchi rounds. Feeding reliability is due to the proprietary MPX polymer and steel-lipped magazines (originally designed in conjunction with Lancer) rather than depending on handgun magazines. The MPX magazines are insanely expensive, but they are very high quality, function flawlessly, and have yet to require replacement.

Another design aspect that contributes to the reliability across a broad range of ammo is Sig’s unique short-stroke pushrod piston design, which subdues the otherwise snappy 9mm PCC recoil of typical blowback guns. Most PCC designs rely simply on a heavy carrier, sometimes a heavy buffer, and a simple blowback design. The result is that most pistol-caliber carbines will seem to have a heavier recoil than standard AR-15s and get picky about which ammo they will function well with. The pushrod piston system smooths out the pressures for consistent reliability across diverse ammo and delivers a less snappy and somewhat cleaner-running firearm. The net is that the Sig MPX piston design can be shot faster with more accuracy.


Product and Aftermarket Support

Sig, of course, has a fantastic warranty and offers excellent “Sig Experience” support for its customers. There have been some reports of a few pre-release guns having some issues with the barrel roll pin loosening, however, I see no signs of that with thousands of rounds through each of these MPX models. The ATF is expensive, slow and a complete pain for SBRs, which have clearly been unfairly demonized as deadly. It is unlikely that a resale of even this well-sought-after Sig MPX SBR would only net a percentage of my original purchase price plus the tax stamp fee. As a consumer, I want to ensure that with this level of sunk cost investment, if I take my toys out to play, they will work and work reliably as expected. The Sig MPX SBR has yet to offer anything but enjoyment.

What I also like about the Sig MPX format is that with the cost of ammo, I can shoot this 9mm format a lot more than the same ammo cost in a .223 or 300 Blackout firearm. Another plus is some AR-15 compatibility of accessories. The trigger is technically an AR format, however, I have beat to death a few standard AR triggers until I switched to bobbed-hammer Hiperfire EDC triggers and PCC triggers. Currently I have run both Timney PCC and HiperFire PCC triggers extremely reliably in two models. There is also the design modularity Sig incorporated in the MPX design, which allows potential barrel swaps to other lengths and potentially other calibers, the handguard swap capability, and the rear picatinny mount that allows a whole variety of stocks and braces to be attached. There has also been a lot of support for everything from carry packs to magazine pouches. Overall, the MPX offers the owner a lot of product support.


Why the MPX?

The PDW (personal defense weapon) and PCC concepts are not new, but the platforms have grown to respond to defensive, competitive and lower-cost training needs of customers, and even the need to just have something fun and new. Sig delivered just that with a highly reliable, adaptable, accurate and feature-rich gun with familiar AR-15 ergonomics. Sig took all the great things people loved about the utterly reliable, short, accurate and super-easy-to-shoot-fast qualities of the timeless 9mm MP5 PDW and then modernized it. Over the last seven years, Sig has created their own legendary PDW.

What initially impressed me most with my first MPX was the Sig MPX SBR actually fed and functioned flawlessly through 4,000-plus rounds during testing. Cleaning and lubrication is expected, but the short-stroke gas piston system is exponentially cleaner and more reliable than blowback pistol-caliber PDWs, which increases reliability. Like all Sig firearms, the MPX is not an inexpensive platform, retailing for $2,000. Even so, despite the fact that it’s selling a PCC with the same price tag as most fully loaded customer precision rifles, Sig still is struggling to keep up with the production of the MPX even years later.

Paired with the Sig branded in-store dealer showrooms, optics, ammo and accessories, Sig is seeing a lot of customers who are becoming “Sig for Life” buyers. The fun and light-handling MPX line is also unintimidating for newer shooters, which has contributed to the sales. Sig made the MPX easy to customize, with full keymod and later M-Lok compatibility, so customers can tack on all the stuff they want to attach, just like their ARs. The picatinny spec rail at the rear of the pistol can be left bare, or with just one torx screw, a folding, sliding or fixed adapter can be used to mount all types of brace/stock accessories and was well-thought-out to afford plenty of DIY modularity.

Accuracy is another feature that will sell the MPX. Retailers with ranges should add an MPX to the rental stable, as this will certainly expand sales once customers can see single-ragged-hole 25-yard groups appear downrange. For dealers that carry Sig optics, the ROMEO series reflex sight is a particularly great pairing for the MPX. The newest factory MPXs models feature exceptional Timney PCC triggers, however, the HiperFire PCC trigger I have tested in my SBR is exceptional for those that purchased an MPX earlier.


Final Thoughts

The current political climate will drive a huge number of braced-pistol-format and SBR sales, so dealers would be wise to add an SOT license to take advantage of sales from NFA-regulated and potentially newly regulated firearms. If there suddenly are new BATFE regulation mandates, some of those mandates could require regular firearms dealers to suddenly become SOT licensed as well.

The Sig MPX is well-thought-out, totally flexible and configurable for future upgrades and for the quality, features, reliability and accuracy — a great investment for customers if they really want a PCC SBR format. This is an investment firearm that is more than just a toy fun gun. This is also a serious piece of equipment that is perfect for home defense, as a mobile package for personal defense, for military/LEO use, and as a simple training tool that anyone can use and train from with no worry of recoil.


