Velocity data is included at the bottom of this post.

After much dithering and confirmation that I’ll still be employed in 2019 (I believe), I decided that I really should have a shorter AR-type weapon, and one chambered for the 300 Blackout cartridge with its distinct advantages over the 223/5.56 round when fired from a short barrel. The SIG MCX attracted my attention due to its action that does not require a long receiver extension (buffer tube) and will therefore fit smaller carrying packages. Because it is legally a pistol, it’s covered by a Colorado Concealed Handgun Permit.
(And yes, I know this is the rifle section, but the gun is more like a rifle in some ways, and SIG lists it with their other rifles, so here it is.)

I ordered the MCX from Osage County Guns, and their service was great. I have a preference these days for long guns that aren’t all black, and therefore chose the FDE version. I had no prior experience with “coat” type finishes, though, and in retrospect if I could do it over, I’d probably try to find a conventional black anodized model. More about that later.

The factory arm brace can be folded to the side to reduce the overall size of the gun, but after looking into the SB Tactical brace, I decided that I liked its adjustable length and switched it for the factory brace as soon as I received the gun despite the fact that it adds a little to the length as compared with the folded factory part.

I am a firm fan of Aimpoint sights, and rather blindly decided on the relatively new CompM5. Despite my research, though, the one thing I overlooked was that despite its superficial resemblance to the CompM3 and M4, it’s a much smaller unit. It also has a 2 MOA dot reticle whereas my other Aimpoints have 4 MOA dots. I was a little concerned if I’d notice the difference in use, but other than having to use a different target for zeroing the M5, I found no handicap in fast and accurate target acquisition and engagement.

The action of the gun is piston driven, my first, but it’s generally the same as the one other piston AR I’ve had experience with. What is significantly different from the usual AR action is that the two recoil springs are positioned side by side above the bolt carrier. As I mentioned, that eliminates the need for a conventional receiver extension and action spring behind the bolt carrier group. As with other piston models, the action remains very clean; after an 80+ round initial session, all I really did was change the oil on the bolt carrier assembly parts. Mostly. The bolt and extractor still required cleaning to remove the buildup of small pieces of brass from the cartridge cases. I obviously could have gotten away with not cleaning those parts for far, far more than 80 rounds, but in time it would be necessary as with any similar gun.

I experimented with three different loads.

Without a lot of research thus far, I have tentatively settled on the Hornady 110 grain V-MAX round for defensive purposes. Reviews of the load fired from 16 inch or longer barrels reported good medium game (deer) hunting results, so I’m hopeful that it will expand reliably even from the short 9 inch MCX barrel. A few early reviews of the MCX reported that some lightweight supersonic bullets evidently disintegrate when fired from the gun’s fast rifling twist barrel, but I experienced no problems with the 110 gn. V-MAX in limited testing. It’s also worth noting that the original 300 BLK MCX barrel had a 1/6 inch twist rate, but the current rifling rate as clearly marked on the barrel is 1/5 inches.

I can only surmise that such a fast twist is designed to reliably stabilize the heavy (long) bullets popularly used in subsonic 300 BLK loads. Although my testing of this and the Hornady 208 gn. A-MAX load were very limited along with the Sellier & Bellot load described below, the Hornady 110 seems to be the most accurate of the three. My accuracy testing was limited by the fact I didn’t really have a good shooting rest, and my primary concern was to get the sight zeroed with training ammunition for a session the same day.

My testing of the 208 gn. subsonic Hornady was also limited to five rounds. The first shot resulted in a failure to eject the fired case. That first 208 grain round, and all the other shots with the two other loads were fired with the gas regulator in the – position as the owner’s manual recommended. After the malfunction, the remaining 208 rounds were fired with the + setting, and with no further problems. As I have experienced with many other subsonic loads, the point of impact was significantly lower than the other tested rounds’, and there was very noticeable vertical stringing at 50 yards. Again, though, the testing was very limited. Changing the gas regulator is very simple; just push a bullet nose or other tool into the hole and rotate the regulator up or down.

The remaining 70+ rounds fired during the initial session were S&B 124 grain full metal jacket. The accuracy was nothing to brag about, but I wasn’t expecting much, especially with my shooting setup. I zeroed the sight at 50 yards, and at short distances the trajectory of the S&B bullet is, according to the Applied Ballistics solver, similar to common 223/5.56 loads. Because I intend for the MCX to be used as a short range defensive weapon, some reasonable commonality is a bonus. Most of the time I shoot my ARs in a multistage “comprehensive skills” course, and although some of my times were a little slower with the MCX, part of that was no doubt due to my lack of familiarity with the gun. Accuracy during the course, however, was as good as I ever expect, including on small, precision shot targets to 25 yards.

As is seemingly becoming more common with higher-tier guns based on the AR system, the MCX has an ambidextrous magazine catch, and I therefore can’t see a way to install my preferred B.A.D.-type bolt catch. I actually surprised myself, though, by having no problems releasing the bolt with the conventional part during fast reloads.

What did give me some trouble was the charging handle and operation. The handle is a quality unit with ambidextrous latches (both sides). Because the MCX receiver is wider just below the latches, however, there is less room to engage the latch when pulling it back. In addition the force required to draw the handle back while compressing the two recoil springs is noticeably greater than with my other ARs. More than once I found myself struggling to retract the bolt fully and wondering if I’d get the dreaded “click” rather than a bang after trying to chamber a round. I’m sure it will not continue to be a problem after I get used to pulling the handle back with more authority, but it was noted.

Above I mentioned the coat-type finish. The gun was shipped with a Lancer 30-round magazine, the first I’ve ever seen in the flesh. The metal top and feed lips are appreciated by many people, but that section of the magazine dragged on the magazine well and the mag wouldn’t drop free when empty. The Lancer mag worked fine in the one Colt AR I tried it with, so I can only suspect that the coat finish of the MCX reduced the width of the interior of the mag well to cause the problem.
The FDE coating is also not very durable. Movement of the rear sling swivel wore the coating away to the base metal in one short session.

Another issue was with 300 Blackout ammunition and some magazines designed for 223/5.56 cartridges. As I’m sure everyone has noticed, the magazines have ridges on the inside near the front that serve to guide the follower. They also contact the shoulder of 223/5.56 cases and limit the forward movement of the rounds. The issue with 300 BLK cartridges is that those ridges contact not the cases, but rather the bullets and therefore push the bullets in toward the center of the magazine. With most magazines that’s not a problem, but it is with some. Heavier bullets with longer bearing surfaces behind the ogive are affected more than, for example, the 110 grain Hornady V-MAX.

For a couple of reasons I decided that I’d mostly use 20-round magazines with the MCX, and I have two types: Brownells GI-type metal mags, and Magpul polymer magazines. The Brownells mags work fine with the 300 ammo, but not the Magpul mag. Why it’s only true of the 20-round mags I have, I don’t know, but ammunition is difficult to load, does not settle into position properly, and most noticeable, it’s about impossible to lock the Magpul magazine into the mag well on a closed bolt even with only a few rounds in the magazine. And after it’s forced into lock, then malfunctions occur immediately.

Magpul does make magazines designed for 300 BLK ammunition, and I have a couple. Other than the markings and some difference in the outside ribbing, the guiding ridges inside are not as tall, and they don’t push on the 300 bullets. Unfortunately, the 300-specific mags are available only as the 30-round version.

Other than those minor issues, I’m quite happy with the MCX.




14 December:

Oehler 35P Chronograph data from range session today. Average measured velocities of 10 rounds of each cartridge (9" barrel). Muzzle velocity calculated by JBM with chronograph 10 feet from muzzle.

Fiocchi 150 grain FMJ
Measured: 1825 fps; calculated MV: 1830 fps

Hornady 110 grain V-MAX
Measured: 2180 fps; calculated MV: 2188 fps

Hornady 190 grain Sub-X (subsonic)
Measured: 1041 fps; calculated MV: 1043 fps


Sellier & Bellot 124 grain FMJ
Measured: 1991; calculated MV: 1998

The Hornady 190 gn. Sub-X was the most precise, about 2.5 MOA at 50 yards. With the sight zeroed at 50 yards for the 110 grain Hornady load, the Sub-X printed about 5.7 inches low. Second most precise was the 110 Hornady, about 3 MOA at 50 yards. The FMJ loads were less precise in general, and both exhibited flyers that opened the groups significantly from where most of the shots clustered.

Keep in mind that the precision figures were with a nonmagnifying red dot sight, and all loads would probably have done better with a good magnifying sight to reduce aiming errors. There were, however, clear differences among the loads. At the end of the session I got five hits with five shots on an IDPA-sized steel silhouette at 216 yards with the S&B load.

This message has been edited. Last edited by: sigfreund,

I've only fired my 8.5" 300 pistol a handful of times, but I'm also quickly noticing a trend in magazines. Magpul and Lancers are not happy, and bulge and stick in my magwell. Magpul 20's work for me, but are extremely sticky with more than 2 rounds loaded.

The only magazines that have been flawless are GI mags, including my straight 20's. I've also moved a bunch of the magpul 20's out of the stash, I don't feel compelled to have so many anymore, I'll probably stick with GI for the next time I order mags.

I stuck with the AR platform but added the LAW folding adapter so that it can stow shorter.

I use 30 round hexmags
Zero issues so far.

sigfreund

Is that the Rattler? What's the barrel length?

James

quote:

Originally posted by depusm12:
Is that the Rattler? What's the barrel length?

No. I’m not very familiar with the Rattler, but this is the MCX Virtus (whatever that is). The barrel is 9 inches.

Added: I just looked at the Rattler, and it seems the differences are barrel length (5.5"), a sliding arm brace rather than folding, and a different style grip.

sigfreund

Lancer makes 300 Blackout specific 20 and 30 round magazines.

James

quote:

Originally posted by depusm12:
sigfreund

Lancer makes 300 Blackout specific 20 and 30 round magazines.

James

That is good to know, but as I mentioned, the one Lancer mag I have will not drop free from my MCX.

I have a Rattler in black. I imagine the lowers must be exactly the same. I have shot mine using 20 round magpul magazines with no issue. I haven't tried the Lancer that came with it. I did try one of the ETS magazines and it didn't drop freely but I have had issues in other rifles with them.
I'll check the Lancer mag when I get home today.

quote:

Originally posted by shoevb:
I'll check the Lancer mag when I get home today.

Did you have a chance to check?

Buy the Lancer 20 and 30 rd magazines in 300 blk. Id rather have a gun that works and have to rip the mag out than a gun that chokes.

I described the problems I had with the Lancer and 20-round Magpuls. I had no problems with anything else and therefore I have a gun that works and beaucoup magazines that work without having to physically remove them from the gun.

But I am always curious about other options.

And besides, it ruins the whole effect when the instructor has to do what he tells his students not to.

I took the Lancer to the range and it functioned perfectly. No problems at all.

Okay, thanks. I still suspect that the slightly thicker FDE coat finish is what causes the problem with the Lancer I have.

FWIW, the Lancers I've tried have all been translucent 30-round. Some clear, some smoke. One would not seat with the bolt closed, even when empty.

Never had an issue with a Lancer in any of my 5.56 guns.

My lower is a CMT and the mag catch is a FCD EMR/A, both of which should be in-spec.

If you don't mind measuring what is the overall length of your set up?

quote:

Originally posted by dewhorse:
If you don't mind measuring what is the overall length of your set up?

With the brace fully collapsed, about 22 3/4".
I did not measure the OAL with the original brace, but with it folded, I estimate about 20 1/2".

Has anyone noticed if there Virtus has a chrome lined barrel? I have the 5.56mm and I am positive it is chrome lined. Sig says they are nitride. What have any of you seen? Thanks

First post updated with velocity data at bottom.

Welcome to the forum, Master Gunny.

I will look at the barrel after cleaning to see if I can tell whether it is chrome lined; doubtful, but worth a look.

quote:

Originally posted by sigfreund:
Fiocchi 150 grain FMJ -- MV: 1830 fps

Hornady 110 grain V-MAX -- 2188 fps

Hornady 190 grain Sub-X -- 1043 fps

The Hornady 190 gn. Sub-X was the most precise, about 2.5 MOA at 50 yards. With the sight zeroed at 50 yards for the 110 grain Hornady load, the Sub-X printed about 5.7 inches low. Second most precise was the 110 Hornady, about 3 MOA at 50 yards. The FMJ loads were less precise in general, and both exhibited flyers that opened the groups significantly from where most of the shots clustered.

Interesting to see your MV data. My data is from an 11.3" barrel:
Fiocchi 150 -- 1881 fps
Hornady 110 -- 2347 fps
Hornady 190 -- 1109 fps

The Fiocchi 150 did 100 yard groups of 2.1" to 3.8". Due to lackluster groups, I didn't shoot it at longer distances. I did determine that its MV was slow enough that I could shoot at my 3/8" steel within 20 yards and not ding it up.

The 110 Vmax gave me 100 yard groups of .8" to 2.7". The biggest issue I had was one high or low flyer every second or third group. At 250 yards I had vertical only (measured this way due to winds in my area) of 2.1" to 5.2". At 370 yards vertical ranged from 2.2" to 8.0".

Hornady 190 gave me 100 yard groups of 2.0" to 3.7". It had lots of vertical stringing at both 50 yards and 100 yards. Hornady's 208 Amax was slightly more accurate, but nothing special.

FWIW my most accurate round is Aussie Outback 125 SMK -- 100 yard groups of .8" to 1.6". Next is Barnes 110 Vortec Tac-TX. Hornady 125 HP American Gunner's accuracy was decent, especially for the price.

Thank you, fritz, for that. Good to compare the same loads from different barrels.