Sig Sauer MCX Pistol with SBX Arm Brace


In the few short years that Sig Sauer has been building AR-15s they have advanced the competence of the platform substantially. The subject of this article is the new MCX Pistol with 11.5-inch barrel and SBX stabilizing arm brace chambered in 5.56 NATO ($1,972).

A solid cheek-weld (and not pressed against the shoulder!) proved our preferred method of firing the MCX.

This isn’t just another AR pistol, but rather the pistol version of a revolutionary new design of the AR-15 platform. The MCX family consists of a 16-inch barreled carbine, both 9- and 11.5-inch barreled SBRs and 9- and 11-.5-inch barreled pistols, with or without the SBX stabilizing arm brace, chambered, variously, in 300 Blackout, 7.62x39mm and 5.56 NATO. Except for barrel length, chambering and stock configuration, they are all virtually identical.

Outside of the ambidextrous magazine release and safety selector, fire controls are where you’d expect to find them on any standard AR-15. Integral and ambidextrous Q.D. rear sling swivel mounts are conveniently machined into the lower receiver.

Perhaps the most outstanding feature of the MCX is its gas piston operating system. Dual captured recoil springs mounted on the operating rod extension above the bolt carrier handle cycling, eliminating the need for a buffer tube altogether. This system has all the advantages of other gas-piston operating systems, but the elimination of the buffer tube permits installation of a range of side folding stocks not available on standard ARs. The side-folding hinge attaches to the rear of the lower receiver, and when folded open holds the stock (or SBX stabilizing arm brace) tight against the left side of the receiver.

Another revolutionary feature of the MCX is the one piece upper receiver and full length Picatinny top rail, machined as a single component. The lower handguard is highly ventilated and has KeyMod flats at the 3, 6, and 9 o’clock positions and fills in the front portion of the Picatinny top rail.

At the heart of the MCX is its dual-captured recoil spring bolt carrier group, which can be thanked for the gun’s reliable operation, and its ability to carry a side-folding stock or arm brace.

On our evaluation gun the upper receiver’s integral Picatinny top rail has 34 slots, and the lower handguard has an 8-slot section, for a total of 42 top rail slots. The 11.5-inch barrel is tipped with a threaded, 3-prong flash suppressor and is rifled at a twist rate of 1:7”.

The MCX proved precise, extremely consistent and dead reliable throughout testing. Our best bench performance came with Black Hills 50 gr. tsx 5.56mm load.

The SBX stabilizing arm brace is attached to the folding hinge and is BATFE approved only as an arm brace, not as a shoulder stock. Made from hard rubber, it provides good traction, and a 1.5-inch wide x 4-inch long flat to bear against the top of your forearm. The 3-inch wide rear portion secures around your forearm with 1.5-inch wide Velcro and does a good job of stabilizing this 5.75 lb. pistol. Although using the SBX brace as a shoulder stock is a big no-no in the eyes of the ATF, shooting it with the brace pressed against your cheek, or with the brace folded and punching the gun forward against the tension of a single-point sling both proved more effective means of of accurate fire.

Controls consist of an ambidextrous safety, ambidextrous magazine release and left-side mounted bolt release, all found in their usual locations. A shell deflector and forward assist are positioned aft of the ejection port. The oversized charging handle has unlocking levers on both sides for truly ambidextrous utility. The trigger is rather nice for a production gun, which is to say better than the typical Mil-Spec variety. It exhibits a barely palpable amount of take-up before breaking crisply at and average pull weight of 4.5 lbs., with zero creep, stacking or overtravel.

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