SIG Sauer MCX Carbine

When SIG offered to send me an MCX rifle configured for police duty use I couldn't wait. MCX rifles feature a slim-profile, chrome-lined, cold-hammer-forged barrel topped off with SIG's own muzzle device.

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Photo: SIG SauerPhoto: SIG Sauer

SIG Sauer has been a strong presence in the law enforcement and military world for decades. The company's firearms are the gold standard for many agencies and their quality and reliability is unquestioned. Although SIG rifles have been popular overseas for generations, the company's bread and butter stateside has traditionally been its line of pistols. That may be changing in the near future though as SIG has recently unleashed two new rifle platforms that are packed with innovations and perfect for duty use.

The SIG MPX rifle was unveiled at the SHOT Show in 2013 and was arguably the most interesting evolution of a submachine gun since the Heckler & Koch MP5. It was the perfect marriage of an AR-15 and the MP5; a rifle designed around proper submachine gun ergonomics but with the familiarity of manipulation and control of an AR-15. Unfortunately back in 2013, we were all limited to hands-on in the SIG Sauer booth as the MPX never made an appearance during Industry Day. Fast forward a few years and the MPX finally hit the market, along with a new sibling, the MCX rifle platform. The years of anticipation paid off and the first MPX and MCX rifles to hit the gun shops were both home runs.

Since their debut I've been fortunate enough to spend quite a bit of time behind the trigger of both rifles and have been nothing short of impressed. So when SIG offered to send me an MCX configured for police duty use I couldn't wait. What made it even better? SIG also sent me one of its new red dot optics, the Romeo 4. I wasted no time mounting the Romeo atop the 16-inch, 5.56mm MCX patrol carbine and was eager to see what the pair could do.

Innovative Design

For those unfamiliar with the MCX, I'll try to break it down for you now. The easiest way to do that is to start with what we all know, the AR-15 platform, and tell you how the MCX has improved upon it.

The 6-pound MCX rifle is similar to the AR-15 in that it is broken down into the upper and lower receiver groups. The forged and machined aluminum lower receiver houses the fire control system and buttstock, although unlike the AR-15 the MCX doesn't need a buffer assembly. This eliminates the need for a buffer tube and allows the MCX to utilize a skeletonized, folding stock. SIG actually offers a few different stock options for the MCX, both folding and collapsible, and they can be easily interchanged in seconds via the vertical 1913 rail mounting point at the rear of the receiver. My example came with the rock solid, folding skeletonized version that was both lightweight and very comfortable.

Moving on to the MCX's fire control system, we see it is very similar to that of an AR-15 rifle. In fact, you can mate a SIG MCX upper to a standard AR-15 lower, if you desire to do so. The difference in the MCX lower is the integration of truly ambidextrous controls. For those of us who shoot with our proper (left) hand this is a welcome addition.

The MCX's safety selector, magazine release, and bolt release are in the same place as an AR-15 receiver so the years of trigger time you've logged on your AR-15 will translate perfectly. SIG has even incorporated two QD attachment points toward the rear of the lower receiver, eliminating the hassle of aftermarket receiver end plates for sling attachments. Throw in an integrated trigger guard and a flared mag well and the MCX's lower half has just about everything you could ask for.

At first glance the MCX's upper receiver looks nearly identical to an AR-15 upper. Take a closer look, however, and you'll see they're almost nothing alike because the MCX's upper contains the majority of SIG's innovations. The MCX operates on a short-stroke piston system; a design SIG has used in some of its other rifles for decades. The gas block is mated to the barrel and houses the piston. It is adjustable for suppressed or unsuppressed configurations and easily disassembled for cleaning. The piston drives an operating rod attached directly to the bolt group. The whole assembly is controlled by two overhead recoil springs and contained entirely in the upper receiver group.

Cleaning and disassembly is incredibly easy with most of the fouling contained to the piston and gas block. This has been the primary gripe about the AR-15 rifle for as long as it has existed and the MCX runs much cleaner. This not only means shortened cleaning time, but also reduces the need for lubrication. The bolt and bolt carrier actually break down almost identically to those of an AR-15, once again shortening the learning curve. Another great innovation found in the upper receiver is a replaceable cam path insert. Anytime you can replace a wear point on a gun, you've instantly extended its service life. That's a nice touch and proof SIG wants its guns to run for a very long time.

MCX rifles feature a slim-profile, chrome-lined, cold-hammer-forged barrel topped off with SIG's own muzzle device. The threads are standard 1/2x28, allowing for the attachment of virtually any aftermarket flash hider or suppressor attachment. Considering this rifle was designed to be suppressed, I suspect more than a few of us will be doing just that.

The barrel mates to the upper receiver and is secured with two robust torx bolts. This means the barrel can be removed for cleaning or swapped out for a different caliber in minutes, even in the field, and all you need is a torx driver to get the job done. This modularity allows the user to switch between barrel lengths for different applications or even change from 5.56mm to .300BLK caliber on the fly. With an AR-15 this could only be accomplished by swapping entire upper receiver assemblies.

Shrouding the barrel on my MCX carbine is SIG's ultra-light skeletonized KeyMod rail system. SIG actually offers this in short and long versions that come in at 8 inches and 12 inches, respectively. The smaller one offers weight savings, but you're also going to give up a little real estate for mounting accessories or support hand placement. I actually preferred the longer of the two and didn't notice the extra couple of ounces. Removal is a breeze; just pull the front pivot pin and slide the guards off the front of the gun.

The front portion of the handguards incorporates a small 1913 rail segment that integrates with the main rail segment on the upper receiver and provides the mounting point for the folding front iron sight. I'll have to admit, I have a hard time trusting any design that allows for the removal of the front sight but the tolerances on the MCX are so precise the rifle held zero just fine after repeatedly breaking it down and putting it back together. I would suggest you test your MCX thoroughly before having such faith, though, as every gun is different and they may not all be as precise.

The Romeo 4

Now let's talk about SIG's new Romeo 4 red dot sight. I've been pretty impressed with SIG's entire line of optics so I wasn't surprised to see the Romeo 4 live up to the rest of SIG's glass. The mere 3.2-ounce Romeo 4 comes in four different variations, the A, B, C, and M; each model has either a CR2032 battery or solar power and different mounting configurations.

SIG claims a battery life of 50,000 hours and if you do the math that is over 5 years of constant illumination. With all my optics, I change the battery once a year just to be sure, and I leave them on all the time. The last thing you want to do in a hasty deployment is remember to dial up your red dot.

The true 1x magnification optic has unlimited eye relief and the 2 MOA red dot is adjustable to 12 levels of intensity, the lowest two compatible with night vision. The glass is clean and clear and the red dot is crisp. Only time will tell how rugged the sight is but my first impressions were very positive.

On the Range

With the optic mounted and a variety of ammo by my side, I hit the range to see what the MCX can do. For my range testing I used four different kinds of ammo: Federal 55-grain TRU Sierra, Hornady 55-grain TAP ballistic tip, Federal XM193 55-grain ball, and Black Hills 77-grain BTHP Match King. I like to throw in a heavy bullet to make sure the gun isn't finicky across the grains.

I dialed in the iron sights first, and then co-witnessed the Romeo with the iron sights using the XM193 at 50 yards. Everything lined up perfectly, and I was shooting tight groups in no time flat, even with what I would consider practice ammo.

Moving back to the 100-yard line, I shot four strings of five rounds each with all four types of ammunition. I averaged 3/4 MOA across the board, with the Black Hills 77-grain shooting the tightest groups of about 0.6 MOA. The short 1:7 twist rate in the carbine-length barrel is the same as the trusty M4 and perfect for heavier bullets. Although all performed well, I would likely lean toward at least a 62-grain bullet for better performance.

Although the gun ran like a Swiss watch with absolutely no function issues, the MCX revealed one Achilles heel during my precision testing; the trigger. I've heard this gripe from more than a few of my fellow firearms professionals, and I have to agree, the trigger is considerably less than ideal. The long creep and gritty break make it very inconsistent; something I look for while timing my trigger control during precision fire. Thankfully with the growing popularity of this gun, aftermarket trigger manufacturers are on board and there now are several drop-in triggers available. If you decide to stick with the factory trigger, I would plan on spending considerable time on the range, both to break in the trigger and get used to the feel of it. I also have a feeling a better trigger would tighten up the accuracy quite a bit.

Switching to more combat-oriented shooting, I moved up to the 25-yard line and ran several magazines through the gun mixed with all four variations of ammo. The skeletonized stock and foregrip allow the majority of the weapon's bulk to be centered over the lower receiver so extended shouldering of this 16-inch gun was effortless.

The Romeo allowed for fast target acquisition and even with the gritty trigger I was able to shoot fast and precise groups from various shooting positions. The ambidextrous controls are very intuitively placed and made for very smooth manipulation, and I didn't have a single malfunction through more than 300 rounds of fire. At the end of the day I was convinced the MCX has a legitimate place amongst the ranks of duty-grade weapons and would take one on duty with me in a heartbeat.

The MCX comes with a price tag of $1,866 although there are enough of them in the market that you can probably find one a bit cheaper. Considering the amount of features and capability that come standard, that is a great deal and the MCX is much cheaper than some of its high-end AR-15 rivals. The Romeo 4 is an even better deal at an MSRP of $399 and can be found for as little as $250 on the web. Compare that to other micro-red dot optics on the market and you'll be saving a boatload of cash.

Take a good look at both at and If you're looking for a new duty weapon and willing to take a step into the next generation this combo is sure to please.

A.J. George is a sergeant with the Scottsdale (AZ) Police Department assigned to the Technical Operations Unit, Special Investigations Section.

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