Key point: There is a reason why it won US Special Operations Command’s Suppressed Upper Receiver Group trials.

The Sig Sauer MCX Virtus could be said to be Sig Sauer, Inc.’s first truly “clean slate” rifle. Earlier rifles like the 556 were derivatives of Schweizer Industrie Gesellschaft (SIG AG) designs, and the Sig Sauer M400 and 516 were both built on the basis of the AR-15, though the 516 sports a piston system.

While the MCX resembles an AR-15 and features a similar layout and many parts, the internal action resembles a AR-18. The MCX also built on Sig Sauer’s earlier MPX in general design cues and in compatibility with aftermarket parts, MPX stocks are compatible on the MCX, and vice versa. As such, the MCX looks to be Sig Sauer’s premier rifle offering, where the MPX will be their premier pistol caliber carbine or SMG offering.

But what makes the MCX different from other piston rifles out there? The MCX is interesting as the design has been proven to work in some very demanding tests. A variant of the MCX won US Special Operations Command’s Suppressed Upper Receiver Group trials, a trial to develop a special drop-in upper receiver that would be able to fire an extremely demanding schedule of ammunition, totally suppressed.

As shown in the Soldier Systems Daily article, the SURG trials rifle also utilized the stock of the MCX, and the upper is just a modified MCX as evidenced by the design of the forward assist, piston visible under the handguard, and handguard design. In effect, the MCX won the SURG trials. Variants of the MCX were also selected as SOCOM’s ultra-short Rattler PDW.

The MCX also was selected as the rifle to arm the London’s elite Counter Terrorist Specialist Firearms Officers (CTSFOs), though first generation MCXs were selected for this contract. However, MCXs suffered some problems with unintended discharge following adoption, though the problems have seemingly been resolved and CTSFOs continue to use the rifle.

But what is the difference between a regular MCX and the MCX Virtus?

Not much. The “Virtus” is effectively the second generation of the MCX with a catchier name. Some minor parts are swapped: the original MCX came in a flat black finish with a Keymod handguard standard, the new MCX Virtus seems to feature a flat grey finish as standard with an M-LOK handguard as standard, reflecting the waning popularity of Keymod. The trigger is also upgraded, with a new two-stage trigger which is lighter and crisper being installed standard on the MCX.

The MCX Virtus also is offered with a telescoping, adjustable AR-style stock as standard, on a folding buffer tube adapter, as opposed to the standard fixed length folding stock the MCX came with.

Now that the issues in the design have been worked out, the MCX seems to be a fairly competitive contender to arm police and military forces around the world, especially given Sig Sauer, Inc.’s aggressive sales strategy of bundling rifles with ammunition, training, and optics to large customers. However, adoption still seems to be slow, with elite police units similar to the CTSFOs making the largest purchases. The Armed Forces of Malta are the only military to adopt the MCX.

Charlie Gao studied political and computer science at Grinnell College and is a frequent commentator on defense and national-security issues.

Image: Sig Sauer.