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AR-15 Semi Auto

Pistol Carbines and The CMMG Guard

by Jim Grant   |  November 16th, 2017   |   0
A great home-defense carbine. This one is topped off with a Bushnell AR Optics Incinerate red-dot and GG&G 45-Degree Transition Sights.

A great home-defense carbine. This one is topped off with a Bushnell AR Optics Incinerate red-dot and GG&G 45-Degree Transition Sights.

Submachine guns are incredibly potent firearms in close quarters and in the hands of a skilled operator. They have fallen out of favor with modern militaries for two major reasons: their limited effective range, and the rise of the short-barreled assault rifle.

These fully automatic or select-fire pistol-caliber firearms offer the individual soldier increased firepower over a sidearm, with better maneuverability than a full-sized rifle in close combat.

Many firearm manufacturers have capitalized on the strengths of these designs, offering civilian-legal, semi-automatic sub-guns in the form of pistol caliber carbines. This idea is not new—Auto Ordnance developed semi-auto versions of its (in)famous Thompson SMGs in the 1970s.

A decade later, H&K engineered an ATF-approved semi-auto version of the MP-5, and RPB Industries developed the M-10 and M-11 open-bolt versions of the MAC-10 and MAC-11 sub-guns. While most of these pistol-caliber carbines were based on famous SMGs like IMI’s UZI carbine, semi-auto-only variants of lesser-known SMGs like the S&W M-76 (the MK-760) also surfaced on the civilian market.

Interestingly, some designs followed the exact opposite path, like the Beretta CX4. Instead of evolving from a submachine gun, the CX4 began as a semi-auto pistol-caliber carbine, before being developed as a select-fire weapon for the government of India as the Mx4 SMG—with more than 36,000 examples manufactured for that contract alone.

The Guard is offered in several different finishes, including this gray Cerakote.

The Guard is offered in several different finishes, including this gray Cerakote.

The reason the idea of a pistol-caliber carbine has survived as long as it has is because the concept of a compact, light-weight, soft-shooting, shoulder-fired firearm are desirable features. In addition, reduced ammunition costs, the ability to shoot in indoor pistol ranges, the reduced muzzle blast, better performance with sound suppressors, increased range over pistols, precision shooting over shotguns loaded with shot ammunition, and the fact that in many cases uses the same magazine as an individual’s sidearm, still holds merit for both civilian, and law enforcement users. The perfect pistol-caliber carbine should enhance the aforementioned traits, not hinder them. However, the downside to pistol-caliber carbines is that they lack the stopping power and long range shooting of their rifle counterparts.

Despite this, many companies producing new pistol-caliber carbines tend to play it safe and simply build direct-blowback firearms. There are outliers, like the KRISS and SIG MPX, but for the most part, if a new design is chambered in a pistol caliber, it’s direct-blowback operated.

While very reliable, the problem with direct blowback is twofold: it needlessly increases both overall weight, and felt recoil. Additionally, the only things keeping the gun in battery during the firing sequence are the bolt’s heft, and the recoil spring tension. This leads to dramatically increased felt recoil because of this heavy bolt’s rapid, unhindered rearward movement during extraction and ejection.

Thankfully, this logic isn’t lost on every designer, which is part of the reason Heckler and Koch found so much success by adapting its 30 caliber, G3 battle rifle’s roller-delayed action to the smaller, 9mm MP5 SMG. Although a tremendous improvement over direct blowback, this design isn’t perfect. For starters, the barrel is pressed into the receiver, requiring either special tools or a professional to replace. Thus, when the system fatigues, the guns require the installation of larger rollers to compensate for the action stretching.

The author firing the Guard at a distant steel target. The soft recoil impulse made plinking incredibly fun!

The author firing the Guard at a distant steel target. The soft recoil impulse made plinking incredibly fun!

This wear and tear wouldn’t be a significant issue if the components involved could be easily replaced or serviced with hand tools. After all, this ease of maintenance/customization, is one of the driving factors behind the AR15’s rise to popularity. Which is one of the reasons I was so excited when CMMG introduced the latest addition to its product lineup: the Guard.

The CMMG Guard, is a radial-delayed blowback, pistol caliber carbine, chambered in .45 ACP. Radial-delayed? Your lying eyes do not deceive, CMMG has pioneered a new method of delayed operation for its 230gr-projectile launcher. The Guard uses a standard AR-15 bolt carrier with a modified carrier key and bolt. The bolt still rotates during the ’unlocking’ phase of operation, though it never really locks in the first place. Where the lugs on a standard AR-15 bolt align with the chamber’s lugs to lock the action shut (directly resisting rearward force from a detonating round,) the lugs on the Guard’s bolt are different. Instead of siphoning gas from the front sight tower and pushing against the carrier’s gas key, the expanding gasses act directly on the bolt face through the barrel/chamber. Because the bolt lugs are cut at 45-degree angles, the rearward force of expanding gasses doesn’t open the action immediately, as friction between the two sets of lugs attempting to scissor past each other slows the bolt’s rotation. Until the bolt rotates, it can’t break free from the chamber, and thus, the carrier can’t move rearward to complete the cycle.

When manually charging the action, the two slide apart without much effort. But under the enormous pressure of a round detonating inside the chamber, the friction is too great for the action to open. Once internal pressure drops to a safe level, the two surfaces slide past one another, and the bolt rotates free of the chamber’s lugs while traveling a short distance back into the carrier.

CMMG’s new .45-caliber guard equipped with Trijicon MRO and StreamLight ProTac tactical light.

CMMG’s new .45-caliber guard equipped with Trijicon MRO and StreamLight ProTac tactical light.

The carrier group then travels rearward, extracting the spent casing, cycling the action and chambering a subsequent round from the magazine (assuming it isn’t depleted.) Which brings me to another solid design aspect of the Guard: its magazines. Older AR-15 carbines chambered in .45ACP often fed from Thompson or Grease Gun magazines. While these are excellent in terms of durability, they are heavy and, unlike in the past, no longer inexpensive.

Instead, the engineers at CMMG chose to feed their newest creation with the most inexpensive, common, reliable, double-stack .45ACP mag on the market—the Glock 21’s polymer magazine. With steel-reinforced feeding lips and a high-impact polymer body, these lightweight mags are available in 10-, 13-, and 23-round varieties. The Guard ships with a single, 13-round Glock OEM magazine, but additional magazines can be found with little effort.

I reached out to CMMG staff to ask why they didn’t include higher-capacity magazines with the Guard, and they responded that the rifle wasn’t always 100% reliable with all aftermarket extended magazines. Interestingly, my testing suggested the opposite, where new-production, 23-round SGM tactical magazines ran without any issues whatsoever.

Another intriguing magazine-related aspect of the gun is the proprietary mag-release button. Even though the Guard is very clearly an AR-15-derived design, it doesn’t utilize a standard magazine release button. Instead, the CMMG carbine uses an oversized, ribbed button that dramatically speeds up reloading times. Shooters with smaller hands will be especially pleased with it, as it is nearly a full inch closer to the pistol grip than the standard AR-15 magazine release.

The Guard’s gray Cerakote compliments its full set of black, Magpul MOE furniture.

The Guard’s gray Cerakote compliments its full set of black, Magpul MOE furniture.

While the magazine release is proprietary, thankfully the charging handle is not. While it’s arguable that the charging handle/latch of the AR-15 isn’t suited to close quarters use, and thus not appropriate on pistol caliber carbines, it does have one advantage. Since the design is identical to that of a mil-spec M16/M4, shooters have a near inexhaustible supply of aftermarket options with which to replace it.

I confirmed this with two of my favorite replacement chargers, the Next Level Armament NLX 556 Ambi Charging handle, and the Devil Dog Concepts Hard-Charger. The NLX product simply gives southpaws and right-handers more leverage on the charger, allowing for easy, one-handed manipulation, while the DDC Hard Charger transforms the host rifle into a side-charger. Neither replacement required any fitting, and both worked as well or better than the factory, mil-spec charging handle.

On the other side of the Guard, the lower features the same bolt-release button as an AR-15. A factory ambidextrous release would have been nice, but since it uses AR-15 mil-spec parts, shooters can simply replace it with something else if they’re not satisfied.

Field-stripping is as easy as with any AR-15-type firearm.

Field-stripping is as easy as with any AR-15-type firearm.

Heading rearward, the Guard utilizes a mil-spec M4 buffer tube, and ships with a Magpul MOE stock in black. A matching Magpul MOE grip resides just forward of the buffer tube and makes for a handsome, yet utilitarian, appearance. Above it, the Guard again borrows from the AR-15, shipping with a standard selector, and mil-spec trigger.

Continuing forward, the Guard ships with a free-floated handguard from CMMG. The 14-inch KeyMod guard features a full-length Picatinny rail on top that blends seamlessly with the fully-railed upper receiver. The two combined result in nearly two full feet of Picatinny rail! This is more than enough rail real-estate for any combination of scope/magnifier/night vision setup.

Lastly, the muzzle of the Guard ships with a modified version of the combination brake/compensator seen on nearly all CMMG’s rifles. In testing, it was tremendously effective as taming what little bark the radially-delayed .45 ACP produced.

Under the compensator, the Guard’s muzzle is threaded to .578×28 RH—the same as aftermarket Glock 21 barrels. I installed my personal SilencerCo Osprey 45 with the appropriate piston to the Guard, and found it to be incredibly quiet with 230gr ball ammo. This was especially the case with Wolf Performance Ammo’s steel-cased 230gr FMJ.

Like all of CMMG’s big-bore rifles, the Guard ships with its combination compensator/muzzle brake atop its .578x28-threaded muzzle.

Like all of CMMG’s big-bore rifles, the Guard ships with its combination compensator/muzzle brake atop its .578×28-threaded muzzle.


The bolt looks like that of a standard AR-15, but a closer look reveals crucial differences, like angled bolt lugs and a solid, truncated carrier key.

CMMG recommends that shooters who intend to run their Guard carbines suppressed do so with an optional heavier bolt. That said, in my limited testing of 200 rounds fired suppressed, I encountered no malfunctions or difficulties as a result of running a silencer on the firearm. The only noticeable differences, other than the shot’s volume, were the noticeable increase in the recoil impulse, and expedited carbon built up inside the Guard’s receiver. Though both of these occurrences are common with any rifle equipped with a sound suppressor.

While the rifle is positively dripping with desirable features, these don’t necessarily make for a dependable firearm. To determine that, I took the rifle home and put it through extensive live-fire testing.

When the rifle first arrived at my FFL, the only ammo I had on hand was a few boxes of Federal’s 185gr lead semi-wadcutter match rounds. Despite the engineers warning me that non-FMJ-shaped projectiles could have feeding issues in brand new guns, the Guard devoured the truncated, light-weight target rounds.

Additionally, the combination of the system’s radially-delayed blowback, and aggressive compensator, made for a yawn-inducingly, soft-shooting carbine. This lack of appreciable recoil, lead to some blisteringly-fast times against my Action Target plate rack.

The Guard ships with a free-floated RKM11 14-inch rail sporting KeyMod slots on three sides and a Picatinny rail on top.

The Guard ships with a free-floated RKM11 14-inch rail sporting KeyMod slots on three sides and a Picatinny rail on top.

At 15 yards, I was clearing all six targets from a low ready in a hair over three seconds. Plate hits were noticeably more positive than with either 9mm carbines or pistols, and I was able to get back on target faster with this carbine than my go-to home-defense short-barreled rifle (SBR,) the SIG MPX.

The next day, when I got some 230gr rounds to test, the lightweight 45-caliber carbine became even more controllable, shaving a few milliseconds from the previous day’s best time. The recoil was so mild, I was compelled to invite smaller-framed shooters to try out the gun.

Everyone from the author’s 105-pound wife, to the next-door neighbor’s 82-year-old grandmother, was able to easily handle the Guard’s gentle recoil impulse.  Most of these shooters also commented on how lightweight the gun was.

The carbine’s modified magazine well and proprietary mag-release are a dead give-away that the Guard is not a standard AR-15.

More experienced shooters marveled at the gun’s lack of heft. Most had experience with direct blowback pistol-caliber carbines that were vastly heavier than CMMG’s latest offering, with one in particular asking about the pistol version.

While I don’t have any experience with the pistol version, it would more than likely make a great basis for building an ultra-handy SBR. Alternatively, with recent rulings by the BATFE allowing users to shoulder pistol braces, the shorter-barreled versions equipped with a brace become even more viable as home or self-defense weapons.

Back to performance.

The Guard was wiped down upon receipt, and given a light coat of Geissele’s ‘Go-Juice’ lubricant. After application, the rifle was fed a total of 500 rounds of varying loads from a pair of factory Glock magazines and two SGM Tactical extended magazines.

During this test, the Guard suffered only two malfunctions—both were failures to feed with Federal Hydra-Shoks. In both cases, the rounds prematurely nose-dived into the feed ramp. Also, both of these malfunctions occurred during the first 50 rounds fired through the rifle. After these malfunctions, the rifle ran without issue for the remaining 400+ rounds fired. This includes a few boxes of mystery (but factory) .45 ACP from a local gun smith’s dump box.

The author’s wife and dog pose with the CMMG Guard topped off with an inverted SilencerCo Osprey 45 sound suppressor.

Toward the end of the shooting tests, the action became a little stiff. This was likely the result of excessive carbon build-up from running a suppressor. A few drops of gun oil on the carrier group resolved this in seconds.

Accuracy was another story. With most carbines, I try to test them at or beyond 100 yards. Given the .45 ACP round’s relatively slow velocity and exaggerated trajectory, I instead moved the target to 75 yards. The rifle was placed on a pair of sandbags atop a shooting bench, and fired in three sets of 10 rounds, with the average being used.

The rifle appears to be built with standard 230gr ball in mind, as it performed the best with that load. The worst performer was Federal’s 185gr FMJ semi wadcutters, grouping 2.6 inches at 75 yards. Other loads produced smaller groups, but all hovered around two and a half inches. While this would be an unimpressive group for a .223 rifle, given the limited effective range of the .45ACP round, this is more than adequate for self-defense or competition use.

This is where the Guard really shines; it fulfills the role of a personal defense weapon for civilians who are either looking for a companion carbine to their Glock 21, or just a hard-hitting-yet-light-shooting home defense weapon. When coupled with a good tactical light, quality defensive ammo and reflex sight, the Guard becomes a force to reckon with.

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