OK, so this one was temporarily souped up. No worries, there’s a factory configuration photo here too.
I think the 300 AAC Blackout caliber was really made for short
barrels. Don’t get me wrong, I have a 300 Blackout AR-type rifle with a
16-inch barrel and love it. I just think the flexibility of the caliber
really shines in short and compact configurations.
With an M4 upper receiver and AR-15 lower, the controls are just as you would expect.
When shooting subsonic, velocity loss resulting from a shorter barrel
is a non-issue, and perhaps even a good thing. I assume, of course,
that you’re shooting with a suppressor in subsonic configuration.
Staying comfortably below that moving target known as the sound barrier
is a benefit when using a silencer as there is no supersonic “crack”
from the bullet.
When shooting supersonic, I don’t think there’s a drawback to short
barrel configurations either. In my opinion, 300 Blackout was never
designed as a long range cartridge anyway. There are certainly better
options to reach out there, like 6.5 Grendel or 6.8 SPC to name just a
couple. Part of the design impetus was to increase terminal performance
over that of 5.56 rounds. 300 Blackout shoots a heavier .308 bullet,
which many believe is a better fight stopper than a lighter .224
diameter projectile. Whatever side of that argument you take, the point
is that a couple hundred feet per second velocity loss from a shorter
barrel really isn’t significant.
A Tour of the CMMG Mk4 300 Blackout Pistol
With all that said, I jumped at the opportunity to put a CMMG Mk4 300
Blackout pistol through it’s paces. I’d shot AR pistols before but had
never really spent quality time getting to know them. What better way to
get acclimated than one chambered in one of my favorite calibers?
One of the nice extras – sling attachment points.
The CMMG Mk4 looks like any other AR pistol at first glance, but when
you examine it more closely, you’ll see some important upgrades. First,
the grip is a Magpul MOE pistol grip. You’ll also notice that the Mk4
comes with a Magpul PMAG rather than some no-name, lowest bidder
30-round magazine. You also might notice the (very) nice KeyMod forend.
That’s the CMMG RKM7 hand guard. It’s got Picatinny rail along the
entire top and KeyMod attachment points at the 3, 6, and 9 o’clock
positions. Each of the three KeyMod rails has six KeyMod holes for
direct attachment of KeyMod compatible accessories or rail segments. My
sample gun came with a two-inch rail segment that occupies three KeyMod
holes.
You’ll also see two quick attach / detach holes towards the back of
the hand guard – one on either side. Speaking of sling attachment
points, you’ll find two sling attachment points between the buffer tube
and receiver. These are small details, but indicative of attention to
important doing it right. When people start talking about being able to
“get X, Y, or Z gun for less money”, I always look for little details
like these that separate one gun from another.
From
the factory, the Mk4 comes with an A2 flash hider. I added this
SilencerCo Trifecta flash hider and suppressor mount to make it easier
to test with and without the silencer.
The trigger on the CMMG Mk4 is a standard single stage, but it feels a
heck of a lot better than most factory AR triggers I’ve ever tried.
It’s got the slightest bit of zero-resistance take up, followed by the
slightest of bumps then a near immediate, and very crisp break. I
repeatedly measured the pull weight at 4.25 pounds with my Timney
Triggers gauge. I’ve gotten spoiled by aftermarket triggers and would
always upgrade at some point, but this one is one of the best “stock” AR
triggers I’ve run across in quite a while. There’s none of the typical
grit that feels like dragging a brick across your teeth. Wow, that hurts
just writing it.
The barrel is eight inches in length and includes an A2-style flash
hider. The twist rate is 1:7, presumably to help stabilize those 220
grain subsonic bricks that you’ll want to shoot from this gun. It’s made
from 4140 CrMo steel with a Nitride finish. Underneath the stock flash
hider, threads are ⅝”-24 and compatible with most any .308 suppressor.
While we’re talking about the barrel, I should note that the Mk4 uses a
pistol-length gas system. We’ll talk more about that later.
Note the texture on the back of the buffer tube.
Moving backward along the gun, you’ll find the standard stuff:
forward assist, standard safety lever, and charging handle. The upper
receiver is forged aluminum M4-type and the lower is standard AR-15. One
thing I did notice is that the rear half of the buffer tube is
textured. I didn’t have the chance to ask the CMMG folks about this, but
I would assume it provides a little extra grip in the event you want to
mount an arm brace or similar device to the tube. Those generally just
slip on, so the texture helps keep things in place.
I might suggest doing something to the gun to scream “this is
chambered in 300 AAC Blackout!” With the short barrel entirely covered
by the KeyMod rail, there’s no simple way to identify the chambering
other than looking in the bore. If it’s your gun, for your use, it’s not
really an issue – just be careful taking it to the range and letting
others shoot without careful oversight. Not too long ago, I saw someone
blow up a 5.56mm AR by stuffing a magazine of 300 Blackout into it while
the owner was otherwise occupied. Clearly, inadvertently putting a .223
/ 5.56mm cartridge in a Blackout rifle won’t have the same result. I
suppose I just like clear markings of chamber type. Just to be clear,
the barrel is stamped with “CMMG 300 BLK” it’s just under the handguard
and hard to see, especially if you have accessories on the gun.
The CMMG Mk4 in its factory configuration.
Shooting a Pistol-Rifle-Pistol
While you can shoot this pistol, or any other AR pistol, as-is out of
the box, it’s a new experience in ergonomics. This is mainly due to the
exposed buffer tube extending behind the pistol body. I did some
shooting with no accessories, sling or brace, just to get a feel for
handling the pistol. My natural tendency, and the wrong thing to do with
this type of gun was to hold it too close to my face. The buffer tube
tends to get in the way and as the gun moves under recoil, it might just
ding your mouth. Even still, holding it two-handed, with no other
support, I could shoot close range targets effectively, say, inside of
50 yards. What really makes an AR pistol shine is using alternative
support. I tried a couple of different methods.
The Sig brace just slides over the buffer tube.
First I added a single point sling, attaching it to one of the rings
between the buffer tube and receiver. The idea of this method is to push
the gun forward against the tension of the sling. This accomplishes two
things. You get stability from the tension and, more importantly, you
don’t have to worry about your teeth getting cracked by the buffer tube.
I shot like this at some hundred yard targets, and while it’s feasible
to hit man sized targets without too much trouble, it’s not really
precise at that range. I think the sling method might be good for
bodyguards, security teams and other short-range scenarios where extreme
precision at range is not really a factor. You can also use a two-point
sling in a similar fashion; you just won’t get the same level of
benefit as using a two-point sling with a rifle also anchored to your
shoulder.
What I really liked, was putting something on the buffer tube to
provide a cheek weld point. Not that you need to hold it against your
cheek to shoot, it just felt more natural to me and provided a better
sight picture with the gun in a bit closer to my body. This led me to…
Shooting with the Sig Sauer SB-15 Forearm Brace
The Sig SB-15 Forearm Brace just slides onto the buffer tube. There
are no permanent or semi-permanent attachments, probably by design.
Between the textured buffer tube surface and a small nub on the brace
that tucks into the receiver, I found that the brace firmly mounted on
the gun. It wasn’t wanting to move around with moderate use. If you
really want to make sure it won’t move, perhaps the classic golf club
tape method would lock things in place. Adding some additional friction
might also allow you to position the brace at the exact position you
want instead of jammed all the way up against the receiver.
Adding the Sig brace made all the difference.
As I wanted to understand the original intent of the Sig brace, I
first used it strapped to my forearm. Soft rubber flaps wrap around your
forearm from the top. Once in place, you cinch up a velcro strap to
hold it firmly in place. This was surprisingly stable! My arm handled
the weight of the gun and if I wanted, I could release pressure from the
pistol grip almost entirely. My firing hand was not required to support
the gun at all – I could focus on aim and trigger control. While I felt
a bit like a Transformer and yes, that was kind of fun, it was a very
effective solution. The only drawback was sight alignment. With the
pistol strapped on to my forearm, the longitudinal sight plane started
farther out to my side, then angled towards the center. Basically, the
geometry of my arm created the angle, so I had to lean my head to the
right (my strong side) to get a proper sight picture. It wasn’t that big
of a deal though.
Once the brace was mounted on the CMMG, I found that even holding the
gun with my cheek on the rubber brace surface made a world of
difference. While any type of cover on the buffer tube, even foam
rubber, would be a big plus, the Sig brace did an admirable job of cheek
and face protection no matter how you hold the Mk4 pistol.
Here the brace is installed partially. Note how it’s not butting up against the receiver.
If you plan on using the Sig brace in any other fashion than it’s
original intent as a forearm support, be very, very careful about
checking the latest ATF rulings. Originally, the ATF deemed that how one
chose to use the brace was irrelevant provided its design intent was to
support one-handed shooting. Recently, however, the ATF has issued an
open letter that appears to reverse their initial stance on the issue.
Sig Sauer and other companies are contesting the ruling, so the final
outcome remains to be seen. Before using this brace in creative ways, be
sure to check on the latest word from the ATF. Don’t rely on this
article or any other for legal advice, be sure to obey all local and
federal laws when you put together your AR pistol with this or any
similar configuration.
Accuracy
Admittedly a bit silly, I mounted a normal scope for some quick and dirty accuracy testing.
I wanted to put this on a rest and test for accuracy just for the
heck of it. I didn’t get ridiculously scientific because this is a
pistol designed for shorter range applications. I brought some
Blackhawk! Sportster Titan rests to the range to do some very quick and
dirty supported shooting. The first thing I found was that getting a
solid support wasn’t all that easy. When testing a rifle, you can rest
the front and back fairly easily. When testing a pistol, the whole gun
can be fairly well supported on sandbags. The AR pistol is somewhere in
between and while I could support the hand guard on a rest, getting
solid support further back was a bit challenging. Much of the problem
was that the Sig brace is made of soft rubber, so putting on a rest
didn’t accomplish much – it wanted to wobble around. I persevered and
tried both subsonic and supersonic loads anyway. Oh, even at risk of
looking somewhat ridiculous, I mounted a full-size rifle scope, a Nikon
P-300 Blackout model, to get some precision at a 50-yard distance. Yes,
it looked a bit silly on the pistol, but I did get a decent sight
picture.
Don’t take these results as any indication of the pistol’s mechanical
accuracy. If anything, the numbers may imply ease of shooting with
accuracy. I was just curious, nothing more, nothing less. I’m confident
that had I bolted this gun to a table, the groups would have been much
smaller.
It’s
hard not to get good accuracy with Sierra Matchking bullets. Even with
an unstable rest, this Gorilla 125 grain Matchking did pretty darn good.
I also got some great results with Hornady’s 110 grain V-Max load.
So the Nikon optic, while great for a rifle, was not the ideal
solution for this pistol. What turned out to be an ideal sighting
solution was the Aimpoint Micro T-2. I had it on the high AR-style
quick-detach mount, and with either sling or Sig brace configuration it
was right where I wanted it. This optic was easy and fast to acquire on
the CMMG. For really close range, the Aimpoint Micro’s tube is small
enough to use as a crude aiming method even without the red dot turned
on. When I actually used the red dot sight, I had no trouble engaging
targets within 100 yards.
Just for fun, I also tried the Trijicon ACOG for 300 Blackout. This
one features a dual reticle for supersonic and subsonic ammunition. The
crosshairs indicate hold points for supersonic loads while two large
dots indicate 50 and 100 yard hold points for subsonic ammo. Like the
Aimpoint, it’s on a higher mount and fell right in line with my sight
plane. Both solutions are ideal for “two eyes open” shooting with a gun
like this.
Function
One thing I noticed – with the pistol-length gas system, this gun ejected brass with authority. Check out the “brass stain.”
300 Blackout guns can be finicky. When designing the gas / spring
balance, four sometimes opposing configurations need to be considered
and all need to work. First, there are two extremes of ammunition and
corresponding pressure: supersonic and subsonic. It’s easy to make a gun
work with one or the other. What’s hard is making the subsonic range
function without beating the gun to death when shooting supersonic with
significantly higher gas pressure. Then you have to account for
suppressor use, which changes the whole dynamic. With a silencer, you
have four possible shooting configurations.
Most 300 Blackout rifle / pistol manufacturers are gravitating
towards pistol-length gas systems and the CMMG Mk4 is no exception.
I did most of my function testing with the CMMG using one of two
different suppressors. Using a suppressor will definitely help reliable
function with the subsonic round, but I felt that was a fair test of
this particular gun. It’s hard to imagine getting a gun like this
without planning on adding a suppressor. On the other hand, I get it.
They’re expensive, and you have to pay the tax, then you have to wait.
If there was ever a gun designed to be used suppressed, this is it.
Trijicon’s 300 Blackout ACOG was an excellent addition as well.
For subsonic shooting, I tried both a SilencerCo / SWR Octane 45 and a
SilencerCo / SWR Specwar 762. The Octane is a pistol suppressor and
only suitable for 300 Blackout subsonic rounds. It’s not made for the
higher pressure of supersonic loads. The Specwar 762 can handle anything
.308, so I used that for all types of 300 Blackout ammunition.
Function was perfect with name-brand 300 Blackout rounds. Only one
ammo type would not function reliably and that was the Cooper Cartridge
245 grain subsonic cinder blocks I had on hand. Well, technically
they’re not really made of cement, they’re lead and have flat points.
Then again, I’ve not gotten these cartridges to cycle in any
semi-automatic gun to date. I suppose they’re intended for bolt actions.
Taking the suppressor off, I was able to encourage a few failures to
cycle with a PNW 200 grain subsonic load. The mainstream ammo worked
fine.
On the supersonic side, the function question was irrelevant. Everything ran like a champ, suppressor or no suppressor.
The Bottom Line
This pistol-rifle-pistol was a lot more fun to shoot than I expected.
Without a suppressor mounted, it’s super compact with its 8-inch
barrel. The flash hider barely extends past the hand guard. Even with a
suppressor mounted, it’s still amazingly compact. At just 5.3 pounds
unloaded, it’s incredibly portable too. It would make a great bug out or
car gun as it would easily fit into transport cases like Blackhawk!
Diversion bags. If I were able to keep this one, the Sig brace would
stay and I’d permanently replace the flash hider with a suppressor mount
version. Other than that, it’s good to go.
MSRP of the CMMG Mk4 300 Blackout Pistol is $1,049.95. CMMG also makes Mk4 pistols in 9mm and .22LR.
I found that the higher mount optics like this Trijicon were just right.
KeyMod attachment points are at the 3, 6, and 9 o’clock positions. The Picatinny rail covers the receiver and hand guard.
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