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Mission-Configurable Carbine: SIG SAUER MCX

sigmcx_1Firearms enthusiasts love controversy.

No facet or feature is too small to elicit a spate of friendly wrangling. For handguns, it is the perpetual "revolvers versus semiautomatics" debate.

Of course, both sides have a winning argument. For rifles, the comparison is of one cartridge to another (.270 versus .30-06, ad infinitum) and different action types reign supreme.

The MCX Patrol Rifle fired for this report came with a 16-inch barrel, a 30-round magazine, an 11.75-inch-long KeyMod handguard, flip-up iron sights, a SIG SAUER textured grip, and a side-folding buttstock.

For the immensely popular modern sporting rifles on the AR-15 platform, a hot topic is the operating system: direct gas impingement or gas piston system.

The well-known gas impingement system has been in use for decades and has earned its stripes in peace and war. Such rifles are accurate and reliable and are used by zillions (well, millions) of happy shooters worldwide.

The so-called disadvantage of the impingement system is that powder gases are blown back into the action, delivering crud that eventually has to be cleaned out.

Guns with the piston system still produce gases and heat, but they vent them in a different place, so the bolt and its carrier are much cleaner after many rounds than with the impingement system. Some experts say that the additional mass of the piston apparatus creates the potential for vibration on the barrel that degrades accuracy. And piston guns are somewhat more costly. However, they are very popular and highly functional.

The SIG SAUER MCX (Mission Configurable Weapons System) is a good example of a piston gun. Shooting Times received an MCX Patrol Rifle for testing, and I put it through its paces on the range.

MCX Features

The MCX rifle from SIG SAUER is a modern sporting rifle that uses what SIG calls a short-stroke gas piston system.

It is available in five versions, identified as the PDW, CQB, Tactical Patrol, Patrol, and Pistol.


All MCX models are chambered for three popular cartridges: 5.56mm NATO/.223 Remington, 7.62x35mm/.300 Blackout, and 7.62x39mm. The features of the MCX vary slightly, depending on the model.

The rifling twist rate is 1:7 for the .223 (1:6 in the CQB .223), 1:6 for the .300 Blackout, and 1:9.25 for the 7.62x39mm.

Screen Shot 2017-04-10 at 2.44.44 PMAll are available with a folding stock — except for the Pistol version, for which a folding pistol brace is available.

The folding stock on ST's MCX was difficult to fold and open, but once locked in place, it'll no doubt stay put. Of course, the folding stock is possible because there is no buffer tube in the stock. Length of pull was 13.8 inches.

The MCX comes with one 30-round magazine, but it accepts all standard AR-15 magazines. The 11.75-inch KeyMod handguard comes with two 2-inch rail adapters for the attachment of various accessories.

There are 13 attachment points on each side of the handguard and 11 on the bottom.

Just for fun, I attached one of the rail adapters to the left side of the handguard and mounted a SIG SAUER STL-100C high-intensity light on it. It slipped on and off in a jiffy and was convenient to turn on with my left thumb. When the rifle was shouldered, it provided great illumination of the field of fire.

All MCX barrel muzzles are threaded. The .223 has a 1/2-28 thread; the .300 Blackout and 7.62x39mm have 5/8-24 threads.

The test gun arrived with an A2 flash hider. The MCX PDW has a 9-inch barrel in all three calibers. A 6.75-inch barrel is also available for the .300 Blackout version of the PDW. The CQB model varies slightly with an 11.5-inch barrel.

The Tactical Patrol comes with a 14.5-inch barrel, the Patrol (sometimes referred to as the "Carbine") has a 16-inch barrel, and the Pistol barrel is 9 inches long. Triggers are listed as "MIL-STD," and the stated pull weight range is 5.5 to 9.0 pounds. More on this later.

The MCX has several noteworthy features only occasionally found on other AR-type rifles. The magazine release and fire control selector are ambidextrous.

Of special interest is the two-position gas valve that controls functioning of the MCX by regulating the amount of gas that is delivered to the action. This adjustment also permits changes in gas volume and pressure for suppressed and unsuppressed operation or for adverse operating environments.

The MCX has several noteworthy features not commonly found on your average AR-15, including ambidextrous magazine release and fire control selector and levers on both sides of the charging handle. Because it's a piston-driven AR, the MCX runs clean. Shown here is the bolt after 100 rounds had been fired.

The adjustment lever is visible through the handguard. Inserting a tool (like the bullet tip of a cartridge) through a hole in the handguard and pushing up or down on the adjustment lever easily changes the gas block setting. There are two settings, and they are detailed in the MCX owner's manual. I put the test sample MCX gas block setting on its "normal" setting, since I did not use a suppressor, and had no problems with any of the 20 loads I fired.

The carbine comes equipped with highly functional flip-up iron sights. These are easily moved or removed, if necessary. Equipping the MCX with all manner of other sights is easy with the flat-top upper and full-length rail with Picatinny slots. Initially, I mounted a Trijicon 3X ACOG on the MCX. So equipped, the rig weighed a hair under 8 pounds.

Shooting Results

I fired several factory loads with the ACOG from the solid rest in my shooting building over the Oehler Model 35P chronograph. Midpoint of the skyscreens was 12 feet from the gun's muzzle. The accuracy averaged 1.89 inches for three, five-shot groups at 100 yards with six different loads. I next tried six handloads, which averaged 2.69 inches, but the average was skewed by four loads with 75- and 80-grain bullets.

I examined the MCX bore, chamber, throat, and rifling with my Hawkeye borescope, and it looked perfect and as smooth as glass. And it is significant to note that after lots of shooting, there was very little jacket fouling. Another boon was the cleanliness of the bolt and carrier after a lengthy range session. After shooting 100 or more rounds, the boltface and carrier were literally as clean as after the last cleaning. That's a big advantage of the gas piston system.

The MCX produced an overall average accuracy of 2.08 inches for 100 five-shot groups fired at 100 yards with 20 different loads. It functioned perfectly during the entire shootout.

At 100 yards, precise aiming was somewhat difficult with the 3X ACOG, so I hypothesized that the low-power sight might be a factor. I had a brand-new Burris Veracity 4-20X 50mm scope with a 30mm tube on hand, so I mounted it on the MCX in a Burris P.E.P.R. mount.

This brought the total weight to 9 pounds, 5 ounces. I retested the six factory loads with the Burris scope set at 8X, and the average was 1.81 inches — virtually the same as with the ACOG. (In fact, a simple t-test showed that the probability of there being a cause-and-effect difference due to the sight change to be much less than 50 percent.) The addition of four more loads brought the overall average for factory ammo down to 1.73 inches.

Again, there was no statistical difference.

Returning to handloads, I retested the same loads fired with the ACOG, plus a few more with a variety of different bullet weights. This resulted in an average group size of 2.31 inches, with the loads with 69-, 75-, and 80-grain bullets producing the largest groups. The MCX's 16-inch barrel has a 1:7 twist, so one would think that heavier bullets would shoot well, but these didn't. Again, the results were virtually the same as with the ACOG sight.

The MCX produced an overall average accuracy of 2.08 inches for 100 five-shot groups fired at 100 yards with 20 different loads. It functioned perfectly during the entire shootout.

Lastly, one cannot discount the heavy trigger on the MCX, which made shooting good groups difficult. The actual pull weight was 9 pounds, 15.5 ounces on a Lyman digital pull gauge, and the range was 11.5 ounces. Factory specs call for a range of 5.5 to 9.0 pounds, so my carbine's pull was about a pound above that limit.

This is not to say that there weren't some good groups produced by the MCX. Cream of the factory load crop was Federal Premium ammo loaded with the Fusion 62-grain bullet. It averaged 1.24 inches, and one five-shot group was a dazzling 0.63 inch.

Another good hunting load was the Federal Premium loaded with the Barnes 55-grain Triple Shock X-Bullet. It grouped into 2.18 inches. This load is no slouch in the game fields, either, as it will shoot through a 300-pound hog's shoulders side to side and exit. I've done it.

Should I wish to decimate a few varmints with the MCX, I'd use the Federal Premium loaded with the Speer 43-grain TNT hollowpoint because it shot very well at a velocity of 3,011 fps. It averaged 1.20 inches. The highest velocity recorded was a sizzling 3,319 fps from the Hornady Superformance load with the 35-grain NTX lead-free bullet.

Speaking of velocities, the average velocity of factory loads from the 16-inch barrel was 13 percent less than the listed velocities.

Overall, the SIG SAUER MCX carbine is a quality example of the gas piston system. Over the course of many range sessions, it never missed a beat, there were no malfunctions of any kind, and it was 100 percent reliable.

The advantages of much less fouling on the bolt and carrier were not unexpected but are noteworthy. It is available in a variety of formats to fit almost any shooting situation and is offered in several larger calibers that are sure to gladden the hearts of field operators everywhere.

While the accuracy results may not set hearts aflutter, this was only one gun. The overall average accuracy of all 20 loads tried with both sights was 2.08 inches.

However, if the four heavy-bullet loads noted above are discounted, the resulting average is 1.96 inches.

For the rough and tumble world of personal defense and combat environments, minute-of-bad guy accuracy, 100-percent reliability, and ease of maintenance are sure to make a good impression.

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