Convert your AR15 to shoot .22 rimfire ammo!

Gerry Veugelaers tells how...

Military training rifles were around well before smokeless powder found its way onto the scene more than 100 years ago, and the .22 long rifle cartridge, that also started out way back then, is still ideally at home in these platforms.

TOP: CMMG 20” upper with 4x IOR M2 scope and Knight’s guards.

Bottom: Gerry’s M4 with home made barrel, modified Ciener kit, 4x ACOG optics, Magpul guards and a Gunworks Canterbury suppressor.

While most training rifles are single-shot, magazine-fed versions also exist, as do adapter kits for modern arms such as the Sig 550 and Steyr AUG. Even the SLR had a conversion available, so an AR15 in .22 rimfire is not exactly a new idea.

Way back in the 1980s, Colt gave away a conversion kit with their A2 sporter rifles to provide cheap practice. My H-Bar (Heavy Barrel) came with one of these kits which used a .223 case-shaped chamber adapter which had a short unrifled section that bridged the gap from the .22 chamber to the .223 rifled barrel. This provided a “jump” of epic (and laughable) proportions, but accuracy in the 1:7 twist barrel was okay for can pounding at 50 yards. Over the years, some dedicated .22lr uppers started to appear but most cost twice that of a .223 upper at the time. What they did do however, was prove that excellent accuracy is possible in a semi-auto despite what the snobs would have us believe.

From the left: A .22lr Ciener 10 round magazine, CMMG 10 round, Blackdog 26 round, and a Blackdog 15 round.

I experimented with various kits and settled on a slow (1:12) twist AR15 A1 barrel that gave me 2” or better groups at 50 yards with most high velocity .22 ammo, and a little better with standard velocity target fodder. It was good enough for practice and plinking but the 10 round magazines of the day were, to put it simply, boring! The real hassle was having to resight your rifle to shoot the .22s accurately, although a spare scope and mount to fit the typical picatinny rail did make this easier. In early 2K, I spent a rainy weekend in my workshop and made up a dedicated H-Bar upper to provide a clone of my usual Service rifle. Coupled with 15 and 25 round magazines from Ciener and Blackdog, I finally had a rig that was not only useful but just plain fun.

Fast forward roughly 10,000 .22lr rounds and dedicated “AR22s” are now fairly commonplace, but the .22lr upper conversion is what is most interesting for me and my Service Rifle brethren given the price of .223 ammo lately.

Using a dedicated .22lr barrel with the correct 1:16 rifling twist makes for a big improvement in accuracy.

I recently imported a bunch of upper assemblies from CMMG in 16” and 20” barrel lengths. The CMMG units are very slick and dead ringers of their .223 parent versions. Gone are the chamber adapters in place of a correct 1:16 twist barrel with integral chamber. The bolt and rails are now stainless steel with magazines from 10 to 50 rounds available.

The whole shipment of 16” M4 look-alikes disappeared almost before they landed but I saved a 20” A2 clone for my own use. Once I had the chance, I unpacked the upper and examined it in detail; here is where the bad news started. The gas block on the M4 units is a screw-locked picatinny unit, but the A2 version is pinned on via two roll pins. Whoever had done mine at the factory must have had a few too many weekbix that morning as when I went to lap the barrel, I could feel the pins slightly constricting the bore at that point. This made little difference to the accuracy but detracted from what was otherwise a very nicely made unit.

I pimped the upper with a set of two-piece Knight’s rails and a spare IOR M2 4x optic in 30mm Nightforce rings. Yes, it was overkill but it did look the business!

A mention of the dark side of .22 kits might now be in order. First, they tend to break firing pins, probably due to the pounding the kits take from .223 weight hammers. The new CMMG units have an AR15-type firing pin retainer, so they are fast to swap out. In those 10,000 odd rounds I have broken three across four kits. One broke after barely 50 rounds, another took several thousand.

This is how much grime builds up after firing a couple of hundred .22lr rounds.

The next bug bear is fouling – .22 ammo produces fouling like nothing else (see picture). The stainless parts make this less of a problem but expect to give your action a good scrub once in a while as fouling can cake up on the bolt face and cause misfires. There’s a lot of scuttlebutt on the net about lead fouling of gas tubes, but an experiment I did some time ago saw around 3000 rounds of the cheapest HV ammo I could buy go through a barrel before I broke it down and checked it from end to end. There was no sign of lead in the gas system, but I have heard of a gas piston gun (Sig 550) bulging a barrel due to a lead plug possibly falling out of the gas port into the bore. If in doubt, clean it out.

TOP: A CMMG collar style for dedicated barrel use.

BOTTOM: A traditional .22lr adaptor for use in a .223 barrel – note the extension at right, shaped like a.223 case.

No doubt the much-abused direct impingement system on the AR15 is better designed for this than piston guns. Next comes feeding. Some kits work with any ammo but mine was not one of them. Federal and Remington ammo ran like a dream but accuracy was mediocre at best. My preferred ammo though, Winchester 555, would not fire at all. CCI grouped well and cycled reliably but was not what I had on hand in quantity so back to the workshop I went.

I found two issues, the first being a sharp chamber mouth that would shave one side of the projectile as it chambered – this explained why the bolt did not fully close at times, resulting in a light firing pin strike. The larger projectiles on the Winchester rounds only made this problem worse. Thankfully this was easily fixed by deburring and polishing the feed ramp and chamber.

The CMMG .22lr adapter is a complete stainless steel unit for use in .223 AR15 rifles.

The other problem I noted was that the headspace on this bolt was considerably more than any of my other kits, although well within specifications. Again, this was easily fixed and once reassembled, Winchester 555s ran like a dream, producing many sub-inch groups at 50 yards, despite the 6lb service trigger on my rifle. Winchester subsonics shot even better but did not always re-cock the hammer so no doubt once run in, these too will work well. CCI subs also produced tight groups and also had the bonus of cycling the action near on perfectly as they seemed a bit punchier than their Winchester rivals.

TOP TO BOTTOM: A No. 7 .22 military training rifle, a CMMG AR15, and a Charger bridge .22.

Note that some breeds of .22 kits do not like notched hammers and can hang up. I have had no such problems with the round nose style so I run these in all my rifles. The same can be said for 9mm kits.

Accuracy was from 3/4” to 2” at 50 yards depending on brand, with the 555s constantly grouping around 3/4”. I set up two small swinging plates, one 4” square, the other 2 x 3”. Both make for challenging targets when shot off hand, especially the smaller plate which is scaled to match the V bull on a Service Rifle figure 12 target. A couple of hundred rounds fired at these make for a cheap and effective practice session without the need to patch targets.

LEFT: A “notched” style hammer.

RIGHT: A round-nosed version. The notched variants can cause bolt hang ups in some kits but the CMMG works with either.

Given the prevalence of such .22s, my local pistol club is planning a .22 action shoot later this year. In all, I reckon these conversions are well worth the effort. My old H-Bar barrel is now converted into a suppressed M4 clone that runs Winchester subsonics perfectly, the Gunworks suppressor making it a very useful hunting rifle. It is so quiet that the loudest things you hear when you fire it are the action closing and the trigger resetting. Set up with a loaned 4x ACOG, a 25 round magazine and a Surefire laser/light combo, a mate’s comment comes to mind, “Bring on those Bunny Zombie hordes...!”


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