No one needs to be convinced that the AR platform lends itself very well to law enforcement. Whether it rides in a marked cruiser, is carried by a SWAT operator or is accurized for a countersniper role, the weapon is now issued or authorized in virtually every department across the nation. Some configurations are tactically better than others. That is why carry handles have given way to flattop receivers and polymer, heat-shielded handguards are being replaced with modular forends. CMMG just released its new Mk4 T series of rifles, and while they can be used to fill several different purposes, law enforcement has to be close to the first.

CMMG has long been one of my favorite companies to visit. Perhaps it is its quirky sense of humor. Any company that designs a muzzle-loading AR rifle or markets “tactical bacon” (hey, everything else has the word “tactical” in front of it), must be a fun place to work. But there isn’t anything funny about CMMG’s new Mk4 T rifles. They are serious rifles with some serious features.

Gun Details

The Mk4 T series is available in four chamberings: .22 LR, 9mm, 5.56mm and 300 AAC Blackout. The 5.56mm and 300 BLK models come with either a 416 stainless steel barrel or a nitrided 4140 chrome-moly steel barrel. Both are offered in what CMMG calls a medium-taper profile. The exterior of the 416 stainless steel barrel is bead-blasted so the metal isn’t shiny. This is done because stainless steel will not accept many surface coatings like Parkerizing or phosphating. The 4140 chrome-moly barrel receives a salt-bath-nitride (SBN) finish both inside and out. This makes the surface of the metal both slicker and harder, which extends the barrel life and makes it easier to clean. The .22 and 9mm are offered with SBN-treated 4140 chrome-moly steel barrels with M4 profiles.

One of the first things that you notice about the Mk4 T platform is the free-floated forend. It is topped with a full-length Picatinny rail that mates up with the Picatinny rail on top of the upper receiver. However, on the bottom and sides of the forend are KeyMod slots. The KeyMod system is a relatively new interface method that allows users to attach lights, grips, lasers and all of their favorite toys while keeping the rifle weight lower than it is when decked with Picatinny rails. This development is taking the AR rifle industry by storm. Forends can now remain more compact and snag free. Attachments now protrude less. And it’s still compatible with all Picatinny-style attachments by simply KeyModding a Picatinny rail into place where needed.

The model that I received for testing was chambered in 9mm. CMMG calls it the “Mk9 T” instead of the Mk4 T for marketing purposes. I have always had a soft spot for 9mm AR submachine guns. I have built, owned and shot more than a few. The first long arm I was ever issued on the job was a full-auto Colt 9mm SMG. In my experience, there is a vast difference in the dependability of these weapons, and I have found more bad ARs chambered in 9mm than any other caliber. In short, if you can make a good 9mm AR that runs, you can probably make about any type of AR rifle that runs.

After the KeyMod forend, the next thing I noticed about my test rifle was the magazine well. Most manufacturers simply put an adapter in the standard AR magazine well so it will fit the double-stack 9mm magazine. CMMG did not take the easy path and simply add an insert into a normal AR lower. Instead, the lower is milled to accept only the 9mm SMG magazine. This feature alone told me that CMMG had taken extra steps to make sure its gun would be of the highest quality and dependability.

The carbine’s other features are pretty standard: an A2-style birdcage flash suppressor and pistol grip, a collapsible stock, a mil-spec trigger and a half-sized dust cover with a brass deflector that you will find on most 9mm AR-pattern submachine guns. CMMG has cut costs by using standardized parts in the areas that are most likely to be changed by the end-user. Lots of people switch out the collapsible stock and remove the pistol grip to replace it with their favorite style. Some people will want a different flash suppressor or muzzle brake. And finally, the list of drop-in triggers available has grown long and illustrious. Whenever a company upgrades these items on its rifles, it costs the consumer more money. And when it is an upgrade the consumer doesn’t want, it needlessly costs more money.

So which parts aren’t standard? The barrel, the forend and the lower receiver. Very few people will change out a barrel. When it is already excellent quality, there is no need. And that lower receiver cannot be changed out. According to the BATFE, that is the serialized part that makes it a gun. As I discussed earlier, CMMG went above and beyond on this part, too. And that KeyMod forend? It represents one of the latest innovations in the AR world. Many AR owners haven’t even heard of it yet. It allows for the attachment of pretty much any accessory yet keeps weight and bulk to a minimum. I no longer know of a single agency that doesn’t allow—and many mandate—a white light on their long arms to help identify who is and isn’t a threat. The KeyMod system is now one of the simplest and easiest ways to attach that white light. Splurging on important features while saving on commonly personalized parts allows CMMG to offer a top-quality weapon with a price set as low as possible.

I picked up my test Mk9 T on my way to the range. However, before I started shooting it, I took the time to give it a quick cleaning and heavily lubricated the bolt. There is no bolt carrier as we know it on the 9mm; the entire piece is referred to as the bolt because it cannot be separated into components like that of a 5.56mm. Also, the 9mm version does not have a gas tube or locking lugs. It is operated by straight blowback. These guns run even dirtier than direct gas impingement weapons. However, if it is a quality weapon and the bolt is kept well lubricated, the gun will run all day long. When you oil the bolt, roll your finger across it so you can see your fingerprints. If the oil runs enough to start filling in your fingerprints, you have enough oil. If it doesn’t, you need more oil. This will save a lot of wear and tear on your gun.

Range Time

To evaluate the Mk9 T’s accuracy, I rough-sighted the gun and ran several different premium loads through it. The one thing I noticed was that the bolt wasn’t locking to the rear after the last round. The magazine I was using had come from CMMG in an open wrapper, and it looked like it had been used before. I wondered if it was simply a bad mag that had been shipped by accident. I tried a different, brand-new magazine from CMMG. No issues. The bolt locked to the rear every time. Then I tried another. Then I dug out some Colt magazines I had brought ranging from first generation with metal followers to more modern ones with plastic followers. They all functioned perfectly. My conclusion: That was a bad magazine, and the gun is good to go. When I mentioned the issue to CMMG later, showing its true customer service, the company offered to replace it, no questions asked. That magazine caused the only malfunctions that I experienced with the rifle.

After the accuracy testing, I got to play. Performing moving-and-shooting drills, reload drills and box drills was nothing but fun. ARs in 5.56mm NATO do not kick much, but 9mms are truly a pleasure to shoot. Whether it is because of less recoil or less noise or less muzzle blast I am not sure. I just know everyone who runs one of these never wants to put it down. Even with the chamfered magazine well, I found I was a little slower inserting magazines than with a regular 5.56mm magazine. Then I started treating the reloads more like the gun was a pistol. After dropping out the empty mag, I rolled the gun outboard and bought it up toward my face. I could clearly see the bottom of the mag well. Sliding the magazine home became much faster and smoother. In no time I was able to make very fast and easy magazine changes while stationary and on the move.

Today, the .40 S&W tends to be the dominant pistol caliber for law enforcement. However, there has been a push recently to move back to the 9mm due to better rounds being available, higher magazine capacity, less recoil and faster follow-up shots. Carrying a 9mm carbine and a 9mm pistol makes a lot of sense. It is easier for agencies to supply the necessary ammunition, and it is easier for officers to manage their ammo when one round fulfills all of their needs. I know I miss my old 9mm submachine gun from years ago. I would love to have the option of carrying one of these CMMG Mk4 T rifles in 9mm, 5.56mm or 300 BLK on duty. They have all of the important features that I am looking for in a rifle, but at a surprisingly reasonable price.

For more information, visit or call 660-248-2293.

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