The .22 LR cartridge has been in use since 1887 and is undoubtedly the most widespread round nowadays. So it is not surprising that firearms that can use this cartridge are built or modified extensively.
The prevalence and low price of this ammo are the primary reasons that so many arms, including the AR-15, are converted to 22LR rimfire.
Advantages of a 22LR AR-15
All gun aficionados know that shooting is a costly hobby and even more expensive sport. Using inexpensive .22 rimfires as a substitute for posh centerfire rounds is one way to create some savings.
Another advantage of the .223 caliber conversion is to allow you get used to the feel of the gun but with less noise and recoil. But you still get the overall look and feel of the AR-15. There are many centerfire caliber conversions available. But the easiest conversion is to the ever-popular rimfire .22 Long Rifle cartridge.
The .22LR is not as cheap as it used to be, but when it comes to price and availability, there's no better bang for your money in the shooting world. For example, for the price of a single .223 cartridge, you can buy 10 rounds of .22LR ammunition.
Another factor is that many indoor or outdoor ranges may not allow you to use 5.56x45mm because of its penetration power and muzzle blast. It’s damaging to backstops and dangerous to wildlife and more importantly people living nearby. But an AR-15 converted to the tiny rimfire will be welcome at all shooting ranges.
Conversion Kit Types and Their Installation
The .22-rimfire conversion units are all pretty much based on the same design. They feature a reciprocating blowback bolt that runs on a guide rail assembly. However they can be divided into two categories: the AR-15 dedicated upper receivers and the Atchisson-style conversion kit.
AR-15 Dedicated Upper Receivers
The complete dedicated .22LR upper is a self-sufficient structure. It installs without gunsmithing and makes a lot of sense for AR-15 owners. It consists of a barrel specifically rifled to work with an all-lead, slower, lighter projectile; the barrel collar, and the bolt assembly. This allows higher accuracy and superior reliability compared to a drop-in .223 to .22LR adapter.
The general build specs of a dedicated AR-15 to .22LR upper are very similar to those of any other AR-15 caliber conversion. Prices starting at around $450 and can run to well over $1,000. In return, you get an AR-15 transformed into an excellent precision shooting machine. The accuracy will be on par with other high-priced rifles in the .22 Long Rifle class.
You end up with a genuine .22LR upper,which is often lighter than original 5.56mm upper receiver group. The weight is lost because there is no need for a gas block or gas tube. Since there is no gas tube to lock the barrel nut in place, you may have to go to longer barrel lengths to make sure it stays tight. Even so, you can get an inch and a quarter shorter overall gun than a regular .223/5.56 carbine.
Most of the .22 conversions and even a high-end dedicated .22 LR AR-15 uppers require a lengthy break-in process and some lubricant (but not too much). After a few hundred rounds, the conversion setup should start running more smoothly. As is the case with most 22’s, you’ll find that your kit may work better with some brands of ammo than others.
Atchisson-style Conversion Kit
A dedicated .22LR upper receiver is nearly as expensive as a dedicated .22LR AR format rifle, but in our opinion, if you’re looking for accuracy it’s the way to go.
Another option is to utilize a drop-in .223 to 22 LR adapter Atchisson/Ciener-style conversion kit. These were developed by the firearms designer Maxwell Atchisson during the early 1970s. Jonathan Arthur Ciener improved them . Because it is the easiest and cheapest way to convert your AR-15 to .22LR, various companies currently produce this kind of .22LR conversions.
Generally, the Atchisson-style rimfire conversion includes only the bolt group assembly and a unique magazine. It does not permanently alter your weapon. You just need to swap out the bolt and carrier group and use a specialized AR-15 .22LR magazine.
The drop-in unit fits all AR-15s and clone firearms of dimensions that correspond to the U.S. Government prints. No tools are required the operation takes all of 30 seconds to accomplish for the average AR-15 owner.
AR-style .22LR magazines have casings of centerfire size. But they have a spring and follower inside to fit the tiny rimfire cartridges. These do not engage the bolt hold-open catch on mil-spec lower receivers.
While the drop-in conversions are simple and allow you to use your existing AR-15 5.56mm barreled action, they typically deliver poor accuracy and reliability. They usually need a higher-power hammer spring, which in turn requires very high-velocity rounds to provide reliable cycling and charging.
Before purchasing any .22LR conversion kit, you should know that these rimfire conversions are NOT compatible with gas-piston-equipped AR-15's.
The CMMG kit is an Atchisson-derived .22 conversion. It holds pride of place in the .22LR conversion-kit field.
This one-piece drop-in unit has improved on Ciener’s design by adding a last-shot hold-open feature to an AR-15. The Bravo Conversion Kit replaces the standard 5.56mm bolt and carrier with a bolt featuring built-in recoil springs. It also has a forward part that looks like a .223 cartridge to bridge the gap from the .22 chamber to the .223-rifled barrel. In fact, the CMMG Bravo kit is intended for 5.56 caliber rifles, not .223 chambered pieces. Also, it’s for direct gas impingement guns only – no piston guns.
As for the accuracy, this conversion is not remarkable, since .223/5.56 is designed for a bore of .224 inches, and the .22LR projectile is made to fit a barrel with an exact .221-inch diameter in the grooves. The other issue that affects .22-conversion accuracy is the much faster rifling twist of an AR barrel (optimized for the 5.56 NATO), which tends to destabilize the slower and shorter .22LR round.
The CMMG .22LR sub-caliber conversion kits come with 26 round capacity polymer .22 LR magazine with bolt catch follower. Be sure to observe any state laws that prohibit the possession of large-capacity magazines.
In any case, these conversion system kits are the most accessible and least expensive method of converting your AR-15 to a .22 caliber plinker. The CMMG kit can be purchased for about $160-$250 and takes about 30 seconds to install.
Building a Dedicated Upper
There is another option that’s a compromise between an overpriced dedicated .22LR upper receivers and the Atchisson-style conversion kit.
While dedicated .22LR Uppers and complete AR rifles in .22LR offer excellent accuracy, they are overly expensive. The price starts at about double of the Atchisson based .22 conversion units. On the other hand, the straightforward and affordable drop-in conversions suffer from accuracy issues, making them suitable only for plinking and recreational shooting.
With the huge availability of aftermarket parts, your other option is to build a custom dedicated AR-15 .22LR upper. It’s simply a matter of inserting a dedicated match-chambered barrel and barrel collar and replacing an entire bolt assembly with an Atchisson-style conversion kit. As an added bonus, you can omit a few parts like the gas block, gas tube, and forward assist.
Doing that, you’ve built a rifle can hold a shooting group just under ½" at 50 yards, which is excellent for an AR-15 .22LR upper making the accuracy ideally suited for hunting varmints and small game with confidence.
Whatever conversion way you choose, using your favorite AR rifle in a smaller caliber with little recoil can provide a new level of shooting enjoyment. The conversion kit makes a lot of sense from a purely economic perspective.
The cheap practice ammo and the ability to shoot at any indoor range make this adaptation even more attractive.