SIG Sauer MCX Review

Thanks to the SIG Sauer MCX's roots, we can rest easy knowing that, while it might be new, it's been thoroughly tested to the tune of tens of thousands of rounds by our nation's elite forces.

SIG Sauer MCX Review

More than ten years ago, I used two rifles daily in Afghanistan. My daytime rifle was a customized AK47 that had its barrel shortened to 8 inches and a side-folding stock. It was super-portable and fired .30-caliber projectiles. Big bullets make big holes, so I carried this rifle as often as the circumstances would allow.

My nighttime rifle was the issued M4 with SOPMOD package that had a longer barrel and could accommodate a suppressor. When set up with an IR laser and used with night vision, this rifle was ideal for low-light use because it offered good ballistic performance while minimizing the noise and light that attracted unwanted attention.

Using two rifles in Afghanistan was a nightmare that required twice as many magazines (cluttering up our vehicle) and a lot of training to retain familiarity with two very different designs. None of us have to do that anymore. SIG Sauer has identified the best characteristics of each of those two rifles, improved them and rolled it all into one tight little package called the SIG Sauer MCX.

SOF Origins

The SIG Sauer MCX project started at SIG Sauer in 2011, but the movement began inside the Special Operations community in late 2009. Kevin Brittingham and Ethan Lessard worked for another company back then, but Kevin was approached by one element within the community and asked if he'd heard of the .300 Whisper.


This unit was buying rifles built for this cartridge and found that less than half of them would work as reliably as needed. They loved the cartridge but wanted a rifle that worked.


From that point on through 2010, guidance on what this element wanted to see took shape. The rifle that would become the SIG Sauer MCX would be similar in size to an HK MP5SD (a suppressed 9mm submachine gun) and had to be at least as quiet. This unit in particular had been using MP5s for several decades, and those guns were 25 years past their life cycle.

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Steel inserts handle the SIG Sauer MCX's high-traffic areas. Here we see the steel cam-pin path insert that prevents the inside of the receiver from getting chewed up. Once worn, it is easily replaced.

Design work on both the new rifle and cartridge proceeded throughout 2010, but in 2011 Kevin and Ethan left the company. Ethan accepted an offer from SIG Sauer, and Kevin, after running his own company for many years, decided to take a break from the industry.

SIG Sauer's work on the project started in earnest in 2011. From the beginning, the SIG Sauer MCX was designed to run with a suppressor. It also had to be military-reliable across multiple calibers. That's a designer's nightmare. Multiple calibers across multiple barrel lengths is hard enough, but when we add the disparate pressure curves that come with using powders designed for both rifle and pistol cartridges in the same semiauto rifle, we present gun designers with the proverbial Gordian Knot.

The SOF guys were looking for a rifle that could handle 5.56x45mm, .300 BLK and, with a new bolt, 7.62x39mm. The interest in 5.56 is pretty obvious, but .300 BLK might surprise some. The reason .300 BLK is popular is that it doesn't experience the radical velocity loss when we shorten the barrel.


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The operating system has dual recoil springs that ride above the bolt carrier and next to the piston op rod. The two springs last much longer than a single heavier spring.

The 5.56 load is heavily dependent on muzzle velocity for good terminal effects, velocity it loses when we chop the barrel on CQB rifles. The .300 BLK loses about half as much velocity when we shorten the barrel, but it also fires a much heavier bullet that helps generate good terminal effects. The .300 BLK is also much easier to suppress effectively and is gaining popularity in SOF circles.

Interest in the 7.62x39mm comes from the occasional SOF need to leave a foreign signature on target and the ability to locally procure that cartridge anywhere in the world. It has slightly inferior ballistics when compared with the .300 BLK but an aggressive case taper that makes it easy to extract.

The tapered case can also be a curse because it gives the long, straight magazine well of the AR-15 fits. Since the SIG Sauer MCX uses a standard AR lower, I asked Kevin Brittingham how SIG Sauer solved the long-standing problem of getting the 7.62x39 to function reliably in a magazine that fits in an AR lower. Many manufacturers have tried and failed.


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Reliability testing of both 6.75-inch- and 16-inch-barrel upper receivers in .300 BLK was performed on a select-fire lower with SIG's suppressor attached. Function was flawless.

"No one has spent the money on R&D and manufacturing [that] SIG has," Brittingham says. "We've developed a new magazine that can handle the case taper."

This is welcome news to all AR lovers because SIG Sauer's 7.62x39 magazine should work in any AR-pattern rifle.

Key Design Points

Cory Newman headed up SIG Sauer's design team. His team focused on making the SIG Sauer MCX light and modular, as per their clients' guidance. They quickly settled on a piston system with dual recoil springs. Piston systems are a great way to regulate the speed with which the action cycles because they can easily bleed off excess gases. When we jump back and forth between short and long barrels and .300 BLK (which uses pistol powder) and 5.56, we would have such wide swings in port pressure with a direct-impingement system that the rifle wouldn't work.

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SIG is loading .300 BLK ammunition that works well in the SIG Sauer MCX. Initial offerings will be match ammunition.

The piston system on the 5.56 and the 7.62x39 barrels have operating rods and gas blocks that share a similar length with carbine-length gas systems on ARs. An operating system of this length is sure to generate enough pressure to cycle the bolt and, thanks to the piston system, bleed excessive gas out the gas block.

The .300 BLK has a system similar in length to pistol-length gas systems. This is necessary for those times when the SIG Sauer MCX will be used with subsonic ammunition without a suppressor. The small amount of pistol powder used in this round doesn't generate much pressure, so the gas block has to be moved closer to the chamber to capitalize on the higher gas pressures found there.

The recoil springs ride above the barrel and make it possible to get rid of the classic AR receiver extension and replace it with a side-folding stock. Much like my long-lost daytime rifle, the SIG Sauer MCX's side-folding stock makes the rifle much more portable than AR variants.

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The SIG Sauer MCX has many stock options. Here we see the very slender side-folder that adds minimal bulk to the rifle.

By using two recoil springs, SIG disperses the load and gives each spring a longer service life than a rifle working off a single recoil spring. This is important on a rifle like the SIG Sauer MCX that expects to see high round counts and infrequent depot-level maintenance.

Other lessons learned and improved upon from the Stoner-designed AR include the use of a steel cam-pin path and steel feedramp. The cam-pin path frequently takes a beating on ARs retrofitted with a piston system. The way Stoner designed the system, gas pressure enters the bolt carrier and pushes it away from the chamber, unlocking the bolt and pulling it and the fired case rearward.

The pressure that moves the carrier rearward also simultaneously pushes the bolt forward into the receiver extension. This forward pressure ensures that the cam pin has enough clearance to rotate freely when the bolt unlocks. When we use a piston system on an AR, we remove the opposing forces from within the bolt carrier and just give it a good smack on the gas key to make it cycle. This results in the cam pin getting knocked rearward into the upper receiver, where it chews its own path through the aluminum.

SIG put a replaceable steel cam-pin path on the SIG Sauer MCX that doesn't wear anywhere nearly as fast as a bare aluminum upper receiver and is easily replaced should the SIG Sauer MCX see enough rounds to wear it out. We still get to keep a version of the AR-style bolt/barrel extension combination that contains pressure so well and lets us build rifles out of aluminum, but it has steel pieces just where we need them.

The feedramp is also a steel insert. During testing, SIG discovered that steel-core ammunition chewed up the feedramp if fed a steady diet of the stuff. Steel-core bullets are widely available and sold in almost all ammunition coming out of Eastern Europe and Russia. These bullets have a thin copper jacket over a mild steel core, and they raise hell with steel targets and aluminum feedramps.

In addition to making several options on side-folding and collapsible stocks, SIG has multiple offerings for handguards, including one with a KeyMod accessory attachment system rather than rails. There are a couple of different lengths depending on which barrel we're using and whether we want to shoot with a suppressor.

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There are two bolts that hold the barrel in place. One maintains rearward pressure, and the other tightens the receiver radially.

One handguard style is very slender and keeps the SIG Sauer MCX svelte and compact. The other style allows for a suppressor to fit underneath the handguard. This gives the shooter the nice, long forend needed for field shooting conditions while still keeping as compact a package as possible, thanks to the short barrel. This setup is excellent for general work, but it can get hot during extended firing sessions.

Snake Charming

After a day at the range with the SIG Sauer MCX, we believe the SIG Sauer MCX will likely find a home with our SOF guys and also be a big hit with civilian shooters. Swapping barrels to change length and/or caliber takes just a few seconds. Removing a pivot pin allows the handguard to come off, exposing the quick-change barrel system.

Two bolts hold the barrel in place. One drives the barrel into the receiver, and the other tightens radially. Both bolts are captured so that we won't lose them. Should we get interrupted mid-barrel change and forget to tighten the retaining bolts back down, the handguard cannot be reinstalled.

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SIG Sauer's BRAVO4 features a horseshoe-dot illuminated reticle. Reticles can be tuned for 5.56, 7.62 or .300 BLK ballistics.

The barrels are hammer forged and nitrided, offering a significant improvement in barrel life over the standard button- rifled/chrome-lined models of yore. Chrome-lined barrels inevitably have variations in thickness, and these inconsistencies cost us accuracy.

Because of the risks associated with using .300 BLK ammunition in a 5.56 barrel, SIG Sauer has slotted the bolt-carrier groups so that the 5.56 barrel cannot be assembled with the .300 BLK-marked bolt-carrier group (BCG) and vice versa. The BCG's markings are visible through the ejection port, providing an important visual indicator showing which barrel is in the SIG Sauer MCX without requiring us to peer down the muzzle. It's hard to look professional when we do that.

The SIG Sauer MCX performed well with both supersonic and subsonic loads. All of our accuracy testing was done with a 6¾-inch barrel chambered in .300 BLK. The best group of the day came from Barnes' 110-grain TAC-TX. Editor Eric Poole managed to place five rounds of this stuff into a .87-inch group at 100 yards. The average group size, however, was 1.73 inches.

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Parting Thought

I've heard some question the need for quick-detach barrels and multicaliber rifles, remarking that no one is going to completely reconfigure his rifle from one day to the next, even inside SOF. That is mostly true, but the need to reconfigure a rifle from one deployment to the next is very real, and all changes have to be done at operator level.

The SIG Sauer MCX makes this scenario a reality, giving military and civilian shooters alike options like we've never had before. Thanks to the SIG Sauer MCX's roots, we can rest easy knowing that, while it might be new, it's been thoroughly tested to the tune of tens of thousands of rounds by our nation's elite forces.

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