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SIG Sauer MCX Virtus Patrol in 5.56 NATO and .300 Blackout: A Modular Masterpiece

The SIG Sauer MCX Virtus Patrol in 5.56 NATO and .300 Blackout can be configured with whatever barrel length, handguards, pistol grips and buttstocks you want, and that's only the beginning of its modularity.

SIG Sauer MCX Virtus Patrol in 5.56 NATO and .300 Blackout: A Modular Masterpiece

(Photo by Mark Fingar)

As usual, it starts with the operators, guys at the tip of the spear whose job it is to run toward the fire. Not only are they the ones who put it on the line to protect our freedoms and the interests of our country, there is also no group better equipped to render judgment about what gear works at the pointy end of warfighting.

Troops and first responders also have specialized needs, and a lot of the development seen in firearms, medical supplies and other gear is the result of their experiences and desire for the best kit possible. Their needs and desires sometimes evolve into a formal request for proposal (RFP), an invitation from the military to manufacturers asking for a solution. Without getting deep into the machinations of government-funded procurement processes, an RFP will often contain guidelines and parameters — some generalized, some specific — that competing entrants must adhere to. Once the submissions are in, products are thoroughly tested and analyzed, often sent back for some refinement. Ultimately a winner emerges and is awarded a contract for production. At least, that is how it is supposed to work.

SIG Sauer MCX Virtus Patrol in 5.56 NATO and .300 Blackout on a Modular Platform
One advantage of the Virtus’ modular design is its ability to be packaged with a variety of interchangeable components. This provides shooters with flexibility in a portable, ready-to-go package. (Photo by Richard King)

Gun enthusiasts have also seen widely advertised RFPs fizzle out before they can cross the finish line. Some will recall the 2013 Individual Carbine’s (IC) premature demise, for example. Recent military RFPs include the U.S. Army’s Modular Handgun System (MHS) trials, which did go the distance and was won by SIG Sauer in January 2017 with its M17 and M18 variants of the P320 pistol.

Another competition that sees SIG Sauer among the leading contenders is the U.S. Army’s ongoing Next Generation Squad Weapon (NGSW) program, which seeks a new rifle (NGSW-R) and automatic rifle (NGSW-AR). These will chamber an exclusive 6.8mm cartridge designed to improve the firepower of small maneuver elements on the battlefield.

SIG Sauer MCX Virtus Patrol in 5.56 NATO and .300 Blackout on a Modular Platform
(Photo by Richard King)

On a smaller scale, elite and specialized units such as the U.S. Army’s Special Forces and U.S. Navy’s SEAL teams also issue RFPs and award contracts for purpose-built hardware. It is from those requests that SIG Sauer’s MCX Virtus was derived in 2015. However, rather than being developed for a narrowly defined need, the MCX Virtus became a user-configurable platform adaptable to meet nearly every small-­arms need of serious trigger pullers. It remains a one-stop-shop for a vast array of mission sets.

The Meaning of “Modular”

The key to the MCX Virtus design is its configurability. Think of the MCX like a ballistic Erector set. It features different parts and pieces that can be swapped in and latched on to change the profile and capabilities of the whole. From handguards to pistol grips and buttstocks to barrels, the MCX Virtus was also designed for multi-caliber use by means of interchangeable barrel assemblies in 5.56 NATO and .300 Blackout.

SIG Sauer MCX Virtus Patrol in 5.56 NATO and .300 Blackout on a Modular Platform
Although countless custom setups are possible, as demonstrated here, Virtus starts out in one of three basic configurations: Patrol, SBR or Pistol. The main differences are barrel and handguard lengths, as well as buttstock options. Sights, suppressors, and other accessories are sold separately. (Photos by Mark Finger)

The most illustrative display of the MCX Virtus’ utility was the development of so-called “assaulter’s kits,” which have been purchased by special operations groups worldwide. The kits can be tailored depending on the receiving unit’s preference. The kit I saw, for example, had a handful of 5.56mm and .300 BLK barrels of multiple lengths with extra handguards to match included along with stock options, both side-folders and telescoping units, and a variety of reflex sights, magnified optics, and suppressors. In the kit was even an MCX Rattler’s upper assembly, and everything was secured and organized in a single Pelican case.

These kits demonstrate how the MCX Virtus can be transformed from a scoped, 16-inch-barreled marksman’s rifle to a concealable personal defense weapon (PDW) in just minutes. And you can’t forget the range of configurations in between. Whether the mission is a sniper’s overwatch, an entry team’s CQB operation, or a plainclothes protective detail, one MCX Virtus receiver can answer the call.

SIG Sauer MCX Virtus Patrol in 5.56 NATO and .300 Blackout on a Modular Platform
Operation of the MCX is different than typical AR-style autoloaders. The bolt is pushed rearward by an operating rod, and is returned forward under the power of dual recoil springs positioned above the bolt assembly. (Photos by Mark Finger)

Another important feature that makes the arrangement possible is the short-stroke piston system in the MCX. Most Black Rifle enthusiasts are familiar with Eugene Stoner’s direct impingement (DI) gas system seen in many AR-15s. SIG Sauer’s approach did away the gas tube and the bolt assembly’s gas key and incorporated a valve to regulate the gas bled from the barrel to actuate a piston and operating rod, which is linked to the bolt assembly. Dual recoil springs run nearly the length of the upper receiver, above the operating assembly, which eliminates the need for a traditional buffer-tube assembly shrouded by a stock. Benefits of SIG Sauer’s approach to the MCX design included the ability to tune the gun’s operation by way of a two-position gas valve. It’s not a major concern with 5.56 ammunition, but an adjustable gas system is incredibly handy for ensuring reliable cycling with the wide assortment of sub-­ and supersonic .300 BLK ammo, and the addition of suppressors. Losing the buffer extension also opened the to door to more stock options such as side-folders without losing the ability to cycle the gun. SIG Sauer’s engineers even thought to incorporate a rail interface at the rear of the lower receiver for attaching stocks or arm braces for popular pistol variants.

SIG Sauer MCX Virtus Patrol in 5.56 NATO and .300 Blackout on a Modular Platform
Besides the trigger, the MCX’s controls include a radial selector, magazine release, and bolt lock/release. The levers mirror standard AR controls in both function and position, except that both the magazine release and selector are ambidextrous. (Photo by Mark Finger)

Commercial Configurations

There has been a lot said about the MCX Virtus’ special-operations origins, but semi­automatic versions have been commercially available since 2015. At this writing, SIG Sauer offers three variants of Virtus: Patrol ($2,200), SBR (Short-Barreled Rifle, $2,430) and Pistol ($2,430). The Patrol comes standard in 5.56 NATO with a 16-­inch barrel rifled at a twist rate of 1:7 inches. SBR variants can be had from the factory in either 5.56 or .300 BLK. In the former chambering, the gun will wear an 11 1/2-inch, 1:7-twist barrel, but is available with a 9- or 5 1/2-inch barrel in .300 rifled at 1:5 inches. 

Remember, though, the SBRs are subject to NFA regulation and require additional ATF paperwork and the payment of a tax stamp. Finally, the pistol, too, comes in either 5.56 or .300, with an 11 1/2- or 9­inch barrel, respectively. Pistol models come with a folding PCB arm brace instead of a stock, but all models mentioned are powered by the same short­stroke piston system, and all the barrels are cold-­hammer forged, which have proven durable and consistent.

Are all these modular parts available to law-abiding citizens? A quick perusal of SIG Sauer’s online parts store ( illustrates that all the configurability offered to those in uniform is also available to us. There are more than a dozen handguards in different lengths and colors, caliber conversion kits for 5.56 NATO and .300 BLK, as well as complete upper receivers. There are even barrels, stocks, arm braces, and the Rattler conversion kit for putting together the most compact Virtus possible.


SIG Sauer MCX Virtus Patrol in 5.56 NATO and .300 Blackout on a Modular Platform
A two-position valve above the barrel allows users to adjust the amount of gas used to power the piston and cycle the action. This can enhance reliability across a range of loads and when a suppressor is used. (Photo by Mark Finger)

And it’s important not forget that SIG Sauer’s optic company offers a complementary suite of models to extract the most out of any caliber or configuration ( In fact, SIG Sauer recently won a U.S. Army contract with it’s Tango 6T 1-6X low-power variable optic (LPVO).

There is some assembly required, but building your Virtus anyway you like is part of the fun with this platform. One caveat, though, and it’s an important one: Mind your barrel lengths and stocks! If you buy a Virtus Patrol, for example, it needs to stay a “rifle” in the eyes of the law. The same rule applies to SBR and Pistol models. Any category changes, if not completed through the proper legal process, could land the unknowing gun tinkerer in hot water.

SIG Sauer MCX Virtus Patrol in 5.56 NATO and .300 Blackout on a Modular Platform
The receiver’s rail interface allows for the attachment of a variety of stocks, including side folders, because the MCX does not use a buffer-tube extension. (Photo by Mark Finger)

The Virtus in Action

A few years back, I had the opportunity to spend two days training with the MCX Virtus at the SIG Sauer Academy ( in Epping, New Hampshire, which is not far from SIG Sauer’s Newington headquarters and manufacturing operations. That event opened my eyes to the concept of versatility through modularity. Training iterations were intentionally varied to showcase the capability of the MCX Virtus platform. 

On the first day, I set up the Virtus as a suppressed 5.56-chambered carbine with a red dot and magnifier to take on drills appropriate for a patrol or general-purpose rifle. Later, I swapped the optics for a variable-power riflescope and experienced the accuracy of the MCX’s cold-hammer forged barrels against targets a few hundred yards out.

On Day 2, things got quiet and then very loud. Back in the classroom, I rebuilt my Virtus into a suppressed .300 BLK carbine and gave it a reflex sight. Then, it was back out for some shorter-distance drills and a culmination exercise in the Academy’s urban CQB simulator, which focused on target identification, fast shooting and transitioning through multiple targets.

SIG Sauer MCX Virtus Patrol in 5.56 NATO and .300 Blackout on a Modular Platform
Barrels, buttstocks and handguards of various lengths are available to customize the MCX Virtus. Barrel assemblies can be had in different lengths and calibers 5.56mm and .300. Railed, M-Lok-compatible handguards are easy to switch and offer additional utility. (Photos by Mark Finger)

I also got to complete a “Jungle Run” with targets ranging from spitting distance to around 200 yards. The key to the exercise was making fast movements between firing positions, but applying calm, deliberate shooting techniques to ensure good hits. For fun the instructors brought out a select-fire MCX Rattler. In .300 BLK, that little PDW is a dream. The Rattler is portable, concealable, controllable, and easy to suppress. In 5.56, though, the Rattler is a firebreather! You can’t reliably suppress a 5 1/2-inch 5.56 autoloader, and the concussion from shooting from inside and around vehicles during testing was the inspiration for its name — it rattled some teeth!

The MCX Virtus is a modular masterpiece bolstered by a core operating system that’s capable of reliable function across an unprecedented range. The Virtus runs without missing a beat. Whatever your shooting needs, from range work to personal defense and professional use, SIG Sauer’s mission-configurable MCX Virtus has everyone covered.

SIG Sauer MCX Virtus Patrol in 5.56 NATO and .300 Blackout on a Modular Platform
Virtus’ modular design is the gun’s banner feature, but cold-hammer forged barrels help the MCX Virtus become an accurate autoloader. (Photo by Richard King)

SIG Sauer MCX Virtus Patrol Specifications

  • Type: Gas-piston operated, semiautomatic
  • Cartridge: 5.56 NATO
  • Capacity: 30 rds.
  • Barrel: 16 in., 1:7-in. twist; cold-hammer forged
  • Overall Length: 35.5 in. (stock extended) 
  • Weight: 7 lbs., 14 oz.
  • Stock: Telescoping, folding
  • Length of Pull: 13 in. (stock extended)
  • Finish: Cerakote
  • Sights: None
  • Safety: Two-position selector
  • MSRP: $2,200
  • Manufacturer: SIG Sauer,
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