Review: CMMG 22LR AR Conversion Kit

Shelling out big bucks for .223 or 5.56 ammunition nowadays can be a bit overwhelming. You know you need to keep training, but, man, those rounds are pricey. So what do you do? How can you keep your shooting skills sharp without spending your life’s savings on ammo? Switching to .22LR has always been a money saver, but you have to buy a whole new gun for that. Or do you? What about shooting .22LR through your AR?

Wait, what? You can’t do that! Well, normally, you’d be right. You can’t just shove a magazine of .22LR into your AR mag well, send the bolt home, and pull the trigger with good results. But what if there was a way to convert your AR into a .22LR shooter and save a ton of money while still training on the AR platform?

In the old days, if you wanted to train on an AR platform but not cough up the dough for the expensive ammo, you had to buy a separate rifle that looked like an AR but was really a .22LR. Companies like Smith & Wesson and others augmented their traditional AR line with .22LR versions that operated the same as the larger caliber big brothers but cost less to shoot. If that’s your thing, you can still get rifles like that. They do offer many benefits, including training on a familiar platform while saving on ammo. But thanks to Missouri-based CMMG, you’re no longer stuck shelling out $400-$600 or more for an AR look-a-like that’s really just a glorified .22LR.

The 22LR AR Conversion Kit does exactly what it says: converts your existing AR into a .22LR practice platform with a quick swap of the bolt carrier group. No more switching from gun to gun.

Early Versions

Early versions of the CMMG .22 conversion kit are a far cry from where they are today. Undoubtedly, CMMG has learned from their own research and customer feedback what needed to be improved on the system. Early versions of the kit had a black phosphate finish and had some break-in period. The current kits have a smooth chrome-like finish that requires minimal lubrication or break-in.

The original CMMG kits shipped with Black Dog Machine magazines and went through several iterations. CMMG currently offers their own magazine. I personally don’t have experience with these newer magazines as my current ones have performed reliably for over a decade.

CMMG offers several enhancements for their .22 conversion kits. A forward assist option is available that allows the user to push the bolt forward should it not fully seat the round. In my experience, this is not an issue, and I currently don’t use it on any of my conversion kits. While nice to have, it’s not a must-have for using this system. CMMG also offers an auto sear conversion that adapts the .22 conversion kit to full auto. In use with full auto rifles, this system can work very well and has an incredibly high cyclic rate. I would estimate in excess of 1,000 rounds per minute. It still makes me giggle to shoot the kit in full auto lowers, as it sounds like a .22 mini gun. Unfortunately, I have had issues with the full auto kit recently, but I attribute this to the introduction of the Geissele SSF trigger I recently added to my full auto lower.

Installation and Maintenance

The CMMG kit is relatively easy to install. Remove the standard AR bolt carrier and replace it with the .22 conversion kit. The conversion kit features a chamber shaped like a .223 casing to adapt it to the AR platform. Close the upper and lower back together and the system is ready to run. No other components need to be swapped.

There is no last shot hold open as the magazine follower is what locks the bolt to the rear. The follower is easily identifiable and lets you know the firearm is empty. Once the magazine is removed, though, the bolt will close. I recommend keeping a chamber flag with the file to keep the bolt open for safety. The operation of the conversion kit is the same as a standard AR platform. The charging handle still charges the weapon to load a round into the chamber. The kit is basically plug-and-play.

For day-to-day maintenance, the gun should be cleaned thoroughly after each use. .22 Long Rifle is inexpensive, but it’s also dirty. After a few hundred rounds, the interior of the upper receiver will be coated in residue and somewhat gritty. As a matter of routine, I will fire a magazine of .223 through the rifle after a .22 range session to verify there is nothing obstructing the operating system (whether direct impingement or piston). The CMMG .22 conversion kit uses direct blowback to function. Since .22 is a lower-pressure cartridge, carbon, unburnt powder, etc., build up around the gas block. Firing the higher pressure .223 will burn off some of this gunk while function-checking the firearm.

I recommend cotton swabs, old toothbrushes, gun solvent, and scrap towels or cloths to detail clean the rifle. This is especially important if converting back to .233, and you plan on using the rifle in a defensive or professional capacity. An AR chamber brush will remove build-up in the chamber and locking lugs. This area tends to collect a lot of debris because the kit doesn’t fill the breech face or chamber toward where the locking lugs engage. A thorough cleaning of the chamber and breech is a must after each range session.

Inspect the barrel for any build-up of lead or copper, as the slower velocity rounds cause additional barrel fouling — especially if you’re using lead. As a matter of experience, copper-jacketed .22 performs better in .22 conversion kits and reduces fouling. Some years ago, a friend borrowed my conversion kit. After several hundred rounds of lead nose through it, the gas port on the rifle was fouled, and the chamber had a build-up of lead around the throat. As a result, the rifle wouldn’t chamber a .223 round. After a thorough cleaning, the rifle was operational again.

After each shooting session, inspect the conversion kit for any damage or wear. My first kit was accidentally dropped on the back side on concrete. This cracked the weld on the back of the kit, and after some welding on my end, it was fixed. Amazingly, the kit ran fine, even with the loose back plate on the conversion kit. I highly recommend inspecting the exterior of the conversion chamber for stress cracks or wear. I’ve experienced two cracked chambers on the conversion kit after extensive use over nearly 13 years on two separate kits. While they still fired, the fouling of the AR’s chamber, throat, and barrel was problematic for cleaning. I have to commend CMMG’s customer service, as a quick phone call and a couple of emails had me with new chambers from them.

How does it work?

The CMMG AR Conversion Kit is super simple to install. Take out your rear takedown pin, remove your existing bolt carrier group, and slide the conversion kit into its place. Close it all back up and off you go! No more changes. That’s it.

The kit completely replaces your BCG and works almost like it. I say almost because it does function slightly differently in some ways but is mostly the same. Some of the differences are internal, and you’ll never notice them. Some are external and might throw you at first but will quickly grow on you.

To convert the chamber from .223 to .22, the kit has a spent .223 casing permanently molded into the end. The fake round sits inside the chamber to direct the smaller caliber round accurately. Yes, they are almost identical but just different enough to need a funnel to direct the round into the barrel. This can cause a bit of a problem, but not always. More about that later.

The conversion kit doesn’t use the buffer tube in the stock for two reasons. One, there’s not enough recoil to need it (it’s .22LR, after all), and two, the kit has a built-in spring that works off the gas system to cycle the bolt within itself.

Shooting the kit is identical to shooting your standard AR with one exception: the bolt catch doesn’t work with it. But that’s really not a big deal because the magazine follower is tall enough to hold open the bolt enough that you’ll feel the lock back and know it’s empty. You’ll still charge the chamber the same as with the normal BCG, but the charging handle only goes back about halfway because that’s all it needs.

Mags & Ammo

As you might expect, the conversion kit requires different magazines, three of which come in the package at 25 rounds each. They look like regular AR mags except for the top. Several manufacturers make .22LR conversion kit magazines, so you don’t have to use the CMMG-provided accessories, but while I didn’t test the kit with others, I assume the CMMG magazines work the best. At least, they worked fine for me.

As mentioned above, the traditional bolt-hold-open function is inoperable with the conversion kit installed, but that doesn’t mean you’ll be clueless when the gun is empty. The bright orange follower sticks up high enough that it holds the bolt open until you release it.

While the magazines worked perfectly, they are not perfectly designed, in my opinion. I would like to see some sort of witness holes to know how many rounds are loaded. Counting to 25 is not hard, but it would be nice not to have to use all the digits on all four appendages and one from the neighbor. Even a single slot at the 25-round mark would be nice to confirm it’s full. Also, these mags are a bit more difficult to load than traditional .22LR mags. Not sure exactly how to solve that, which is why I’m not an engineer, but I bet someone smart at CMMG (there are a lot of smart folks there) could figure it out.

If you’ve shot .22LR long enough, you know two things about it that most accurately describe the typical small-caliber shoot experience. First is that .22LR is notorious for being a dirty round. It creates a huge amount of carbon for its size. It’s hard to imagine how such a small round can make guns so dirty, but it does. The second is that .22LR hates shooting out of a dirty gun. Ironically, the same caliber that hates the dirt creates the dirt.

Compounding the challenge is that shooting .22LR out of a .223 barrel is a mismatch, so it’s shooting out of a gun it’s not designed for. All of these factors resulted in a very disappointing first time out with the kit. About a third of the rounds jammed, misfired, failed to feed, failed to eject, etc. Frankly, I was shocked. I expected some level of poor reliability because .22LR by nature is unreliable. It’s one reason you should never carry it for self-defense. But I didn’t expect it to be this bad.

Two factors contributed to the bad outing, both my fault. Since the conversion kit was straight out of the box, it was not sufficiently broken in. The recoil spring was tight, causing the system to not work as efficiently as it was designed. Also, I was using cheap bulk-pack ammunition, the kind you can get for cheap because it’s cheap. It caused all sorts of problems, as chronicled above. After the initial break-in period of about 100 rounds and getting wise to the bulk ammo issue, I switched to CCI Mini Mags, and the experience from that point forward was night and day better. The system cycled properly with far fewer stoppages and much better shooting consistency.

When I got it home, I cleaned it out of habit and because I wanted to see how dirty it got. As expected, it was filthy. I ran a bore snake through the fake chamber cartridge, and it came out black and powdery. The entire unit was caked in ugly carbon inside and out. However, a little rubdown with gun-grade solvent did the trick. Soon, it was squeaky clean. Most calibers don’t require regular cleaning, but I recommend cleaning the conversion kit after every use to keep it running smoothly. It is a .22LR, after all.

I took the conversion kit out several more times over the following weeks—for science, of course—and had much better luck. Breaking it in and changing to better ammo did the trick.

Overall Impressions of the CMMG 22LR Conversion Kit

While my relationship with the CMMG 22LR AR Conversion Kit got off to a rocky start, it got much better once we figured each other out. If you want to save a bunch of money and not have to buy two guns, this AR conversion kit is the way to go. At just $229.95 for the drop-in BCG and three 25-round magazines, it’s a whole lot less expensive than an entire gun. And it lets you train on a rifle you’re already familiar with. My recommendation? Order yours today.