I’m interested in trying one of these out. Any advice or recommendations for a conversion kit? Thanks in advance


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I have one made by CMMG I purchased several years ago. No problems so far.

Best AR-15 to .22LR Conversion Kits in 2021 | Peak Firearms

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Ret. LE, FA Instr, S&W Armorer

I have one of the CMMG ones too. was having failure to fire problems and they really worked with me. sick thing was that a single lower/upper combination would have the issue, regardless of the trigger parts used, but either the upper or lower, when paired with a different upper or lower, ran fine! I finally tried bending a trigger spring a bit the "wrong" way and the problem combination then worked fine! Bottom line is that CMMG really went all out to help me with the problem. At least with an old vietnam era m16 1 in 12 twist barrel, the accuracy is quite good considering what it is. around 1 1/2" at 50 yds.

The issue with any of these is that the twist rate will be wrong. The old VN-era M16 has a 1 in 12 twist. That's close to standard 1 in 16 twist rate for a .22LR round. Most modern .223/5.56 barrels are 1 in 7 to 1 in 9. The M&P 15-22 twist rate is 1 in 15 for example.

I should have mentioned that I had first tried it in a fast twist upper and accuracy was really mediocre so having an old m16 barrel with triangle grips lying around, I tried that because of the twist and was happy with the result. But i had the old barrel and grips just lying around....

The CMMG and the Ceiner conversion units are both of the Atchisson design. They work pretty well, but like any .22 rimfire they are very dirty. The AR lower receiver will get dirtier sooner than it will from shooting 5.56mm.

I used a Ceiner unit for years on my Bushmaster AR15 in my backyard range. It has shown excellent reiliability. The difference in rifling rate between .22 RF and 5.56/.223 is often cited in discussions on this topic, as this affects accuracy of the rimfire rounds. Practically speaking, if I do my part I can hit a 6-inch steel gong offhand at 70 yards 7 out of ten times on a good day. That's plenty good enough for me.

Far more significant limitations are first, that the rifle must be throughly cleaned after putting several hundred rimfire rounds through it, because the centerfire chamber -- into which the rimfire adapter must be inserted to sh0ot .22LR -- may get too dirty to reliably chamber the centerfire ammunition. Second, the sights will have to be re-zeroed since the points of impact will vary significantly between centerfire and rimfire.

Full disclosure: I now shoot a dedicated CMMG .22LR upper to avoid the issues mentioned in the above paragraph. Accuracy is improved as well. But the bottom line is that the conversion units are well worth their initial cost if you want to shoot your AR frequently. If you do go this route, buy Black Dog magazines and a Lightning Loader from McFadden machine.

Oh, and I'll add Semper fi, Devil Dog. Although a sailor, I spent 6 years in the FMF in 2d ANGLICO.

I also have a CMMG kit, but mine is a fully dedicated rifle... .22lr upper on a multi caliber lower, but purchased as a complete rifle. I added a Redi-mag S&W adapter to mine to allow me to use the M&P 15-22 magazines, as well as have the bolt hold open after the last round in the magazine. I chose the S&W magazines over Black Dog because the S&W mag was readily available at stores like Academy and Cabela's.

Better-Mag S&W(R) .22 Rimfire Magazine Adaptor - Redi-Mag

While for my own reasons I prefer the Black Dog magazines, it is worth pointing out that the S&W magazines will function the bolt hold open (BHO) without the extra effort required to make the BD mags do that.

Another vote here for the CMMG kit. I run mine in 5.56 chambered Stoner upper. I also purchased a couple of accessories from Bore Buddy .. the charging handle insert and the buffer tube plug. The former helps reduce the blowback gases hitting me in the face. The latter helps reduce "bolt bounce".
In one particular range session I decided to "stress test" the system. I fired off over 700 rounds of .22 through the CMMG ... other than one stovepipe, the damned thing was flawless. One caveat .. when using the CMMG kit, I only use jacketed/plated/poly-coated "High Velocity" ammo.

I got a CMMG Kit and took it to the range today. Used CCI 40 gr MM, Remington Golden Bullets and Federal Bulk Pack. Worked like a charm, will pay for itself soon.