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History might not always repeat itself, but it often rhymes. Taking a quick look around the gun store will leave a casual gun shopper overwhelmed with the number of calibers an AR rifle is available in these days, but of note is the resurgence in the popularity of pistol-caliber carbines (PCC).

Whether the proliferation of suppressors, innovative pistol braces or the effectiveness of new handgun-caliber cartridges are to blame for the newfound interest in PCCs can be left to the Internet discussion boards. What’s not debatable is the significant jump in quality and reliability of the AR-based PCCs available today compared to what was available a mere decade ago.

So, when asked if I’d like the opportunity to take a look at the latest offering from CMMG that is chambered in .45 ACP, I jumped at the chance. Unlike the devout disciples of John Moses Browning, I am admittedly not slavishly devoted to the .45 ACP cartridge.

What made me interested in the CMMG MkG45 GUARD line was not the caliber—and it wasn’t even the CMMG reputation for quality; rather, it was the unique way CMMG approached the PCC operating system.


For a bit of background, let’s take a look at the different ways a PCC can be made to work. Until recently, AR-platform PCCs were limited to either blowback or direct impingement (DI) methods of operation, with roller-delayed blowback and other options being left to Heckler & Koch and other weapons systems. What makes the CMMG GUARD line unique is the newly patented, radially delayed blowback system CMMG introduced for its PCC offerings.

The MkG45 bolt (center) is nearly identical to the standard AR-15 bolt (left), except for the obviously larger bolt face. The blowback .45 ACP bolt (right) is also shown to compare the MkG45 to the bolt found in most of the .45 ACP rifles on the market today.

Traditionally, a PCC utilized a straight blowback method of operation. In a blowback weapon, a heavy bolt and a heavy spring were the only things keeping the action closed upon a chambered round. The blowback weapons relied on a large, reciprocating mass to function, and it resulted in a weapon with pronounced recoil and a  “choppy” feel. Look no further than an M3 Grease Gun or M1928A1 Thompson for examples of legacy .45 ACP weapons that utilized straight blowback operation.

The standard AR uses the direct impingement method of operation: A small portion of the gas from a fired round is directed back into the action to unlock and cycle the bolt. This requires a relatively uniform volume and  velocity of gas to be directed back into the action and results in a lighter bolt, buffer and spring. However, it’s  dirty and can be difficult to get right with a pistol-caliber round that has a differing pressure curve from rifle rounds. Direct impingement also suffers from rounded and truncated projectiles that have to feed between the locking lugs of the barrel extension. While direct-impingement .45 ACP carbines are not unheard of (Macon Armory produced one of the more reliable and high-quality conversions I’ve seen), they can be finicky and difficult to get right.

The feed ramps of the MkG45 are similar to those of the AR-15 and are visually identical to those of the direct-impingement .45 carbines. (The Macon Armory DI barrel and extension are shown here, because companies frown on writers tearing down demo guns to this degree.)

The CMMG GUARD MkG45 is neither fish nor fowl. It is not straight blowback; nor is it direct impingement. It is a new method of operation in the AR-15 envelope: a rotary-delayed blowback. Using a rotationally-locking bolt (as do direct-impingement weapons) and a heavy carbine buffer, the CMMG does not rely on recycled gas from the fired round to unlock and cycle the action. Instead, a spring-loaded bolt is coupled with angled bolt-locking lugs to produce a PCC with a low recoiling mass. This allows for reduced perceived recoil and a faster follow-up shot than the straight blowback options while also eliminating the gas system of the DI system options; thus, the resultant fowling of the action brought about when exhaust gases are cycled into a mechanical system.

“This smooth-shooting rifle was fun and an all-around joy to shoot. The fit and finish were what I would expect an upmarket rifle to be—and it was more accurate than I was when shooting in the field.”


When you crack open the MkG45 and field-strip the rifle, the most interesting points of observation are in the bolt carrier group (BCG). The bolt face looks like an oversized 5.56mm bolt with eight locking lugs and an  identically placed extractor and ejector. If you check out the rear of the bolt, the pronounced difference becomes more apparent: The rear faces of the locking lugs are angled, leaving a slight lip to engage the barrel extension. During the locking step in the cycle of function, the bolt head is cammed into the locked position, leaving that slight lip on each lug the only surface contacting the barrel extension. This provides a minimal friction surface to impede the unlocking of the action, which would otherwise require a direct-impingement gas system to accomplish.

The radially-delayed blowback action is made possible by the angled lugs on the rear of the bolt, shown on the left.

For the bolt carrier: Where a gas key normally appears on a standard AR bolt carrier is instead a machined bolt carrier extension that is cut to accommodate the round cam pin—instead of the traditional rectangular one. When disassembling the BCG, the only difference from a standard AR is the small spring that assists the bolt in unlocking the action after a round has been fired.

Finally, the BCG has the same profile on the bottom as a full-auto M16 BCG and will function with an NFA-registered drop-in, full-auto sear. The rear hole in the carrier is unique to the CMMG GUARD firearms, and it is to accommodate the optional action-tuning kit to add weight. This is to allow the shooter to adjust the weight of the bolt to accommodate +P rounds and the use of a suppressor, both of which can change the operation of the rifle, thereby causing malfunctions.


When I pulled the rifle out of the box, the first thing that caught my eye was the finish. The silver Cerakote is a factory option that will add $150 to the price tag. Nevertheless, it is very well done and was the first thing people commented on when seeing it at the range. The billet lower is a beauty, incorporating a flared magazine well and integrated trigger guard, as is common on many high-end receivers.

The MkG45 is available in 16.1-inch barrels for the rifle configuration and 8 inches for the SBR and pistol configurations. Even with the addition of a suppressor, the rifle is not overly long.

CMMG really nailed it, though, with its magazine release design. PCCs that use Glock-pattern magazines usually require a magazine release in the front of the magazine well. However, the CMMG uses a simple lever to engage the magazine notch. It has a very large and well-placed surface to which the trigger finger reflexively indexes, providing quick and effortless magazine changes. Other manufacturers have accomplished the Glock-pattern magazine release in similar ways, but this is the hands-down best I have seen so far.

“CMMG really nailed it… with its magazine release redesign.”

Easily the author’s favorite ergonomic feature of the MkG45 is the magazine release. The wide surface falls right under the trigger finger when indexed along the receiver. The magazine release locks up tightly with all the Glock-pattern magazines he tried.

In the same vein, the last-round bolt hold-open mechanism in the lower receiver is well done and functions reliably with the Glock magazines that were used for the testing. The rest of the rifle is “boringly” high quality, as one would expect from CMMG. The fit, finish and manufacturing were on point and left me eager to see how well the rifle shot.


Not content to subject this rifle to an air-conditioned indoor range with great lighting and no wind, I dragged the rifle out to a secluded spot in the Sonoran desert, nine miles off a paved road. With a full-value 10-mph wind and temperatures knocking on triple digits, I was able to put nearly 400 rounds through the rifle with only the minor ammunition-related issues I will discuss later. Given the nature of the .45 ACP round, I opted for a SIG ROMEO5 red-dot with a 2 MOA dot and performed all my firing at 25 yards. And while the results will certainly not take me to Camp Perry, they are respectable—given the conditions and my mediocre marksmanship. The five-shot groups hovered around the 2-inch mark, with the best being 1.38 inches from DoubleTap’s 160-grain +P TAC-XP.

“… if suppressing this rifle is a consideration, you should give the ammunition a try before assuming it is subsonic in the longer-barreled rifles.”

Trying a range of projectile types, the rifle digested standard FMJ and FMJ-FP rounds without a hiccup but found an occasional JHP round that would fail to feed. Due to the projectiles’ geometry and the feed ramps that have to be cut through the barrel extension, the pistol-caliber carbines can be very specific with regard to which jacketed hollowpoints will feed reliably.

The SIG ROMEO5 red-dot paired quite nicely with the rifle. Ten illumination settings allow the optic to be used with night vision or on a cloudless desert afternoon without being washed out. With a motion-activated illumination system, the single battery can last up to five years.

For this evaluation, the 230-grain SIG V-Crown JHP ammo had an occasional issue, while the 160-grain DoubleTap +P TAC-XP had none. Also worth noting: With a 16-inch barrel, there were even 230-grain bullets that would go supersonic consistently. So, if suppressing this rifle is a consideration, you should give the ammunition a try before assuming it is subsonic in the longer-barreled rifles.

The MkG45 ships standard with a single 13-round magazine but will accept any Glock-pattern .45 ACP magazine, including the extended .45 ACP magazines used by the Kriss submachine gun.


This smooth-shooting rifle was fun and an all-around joy to shoot. The fit and finish were what I would expect an upmarket rifle to be—and it was more accurate than I was when shooting in the field. That said, I couldn’t help but want a shorter rifle, especially when the suppressor was attached. In states where it would be lawful to do so, the 8-inch MkG45 would be my ideal PDW when married to the Omega 45k from SilencerCo. And, as with many pistol-caliber carbines, ammunition selection hinges on trial and error, especially when suppressors are involved. In short, if you are in the market for a .45 ACP rifle, you can’t go wrong by buying the CMMG MkG45 GUARD.

This rifle shines when it is wearing a suppressor. The SilencerCo Omega 45k proved ideal to suppress the MkG45 and was noticeably quieter on the radially delayed blowback action than on a blowback .45 ACP rifle. The Omega 45k requires an additional fixed barrel spacer and .578×28 piston for use on the MkG45.





SIG 230-grain FMJ



SIG 230-grain JHP V-Crown



DoubleTap Tactical 230-grain FMJ-FP



DoubleTap Tactical 160-grain +P TAC-XP



Colt Competition National Match 230-grain FMJ



NOTES: Groups were taken from five five-shot groups fired at 25 yards. Velocity was taken from five shots using a Caldwell Ballistic Precision chronograph. 




BARREL: 16.1 inches; 1:16 twist; medium-taper 4140 SBN barrel; threaded .578-28 (includes CMMG SV Brake)

FURNITURE: CMMG RKM14 handguard, Magpul MOE pistol grip, Magpul CTR stock with six-position MILSPEC receiver extension

RECEIVERS: Billet 7075-T6 aluminum lower; forged 7075-T6 aluminum upper

TRIGGER: CMMG single-stage MIL-SPEC-style trigger

WEIGHT: 5.8 pounds (unloaded)

LENGTH: 32.5 inches (with collapsed stock)

OPTIONS: Action Tuning Kit, $24.95; Cerakoted upper, lower and handguard, $150

MSRP: $1,400



Editor’s note: A version of this article first appeared in the July 2018 print issue of Gun World Magazine.