Building a 22LR AR Pistol
© 2022 No4Mk1T
© 2022 No4Mk1T
I built this when I was off between Christmas and New Years. Originally posted on the NFA forum I mod.
Been thinking about building a AR rimfire pistol. By fortuitous happenstance, I spied that PSA has a dedicated RF pistol upper on Christmas sale for less than the parts to build it myself. So I bought it.
Being this will be used suppressed 99% of the time, functioning was tweaked to that end.
Used one of the 80% lowers I had on hand waiting for a build.
Ordered all the necessary parts, the last of which arrived today.
Running S&W mags for the ease of loading aspect, and also because the follower design allows for LRBHO when coupled with the Redi-Mag adapter so it's just like CF.
The CMMG RF bolts are known for bolt bounce, so upgrades from Bore Buddy were installed to mitigate that. Also firing pin and extractor upgrades.
Got it assembled just in time for a quick 5 rnds suppressed of CCI SV out back before it got dark. No malfs, and the bolt locked back.
80% lower with retro ArmaLite markings for the cool factor, and to troll the anti GG peeps. To the unknowing, it's a factory lower.
The Pile-O-Parts. The fun begins...
Finished.
Well, it works...now.
To be upfront, the first range trip, I ran it as received.
It did function, but there were enough issues to make for a frustrating experience.
- At least six instances of a case getting jammed above the BCG causing a stoppage.
- Several (a dozen or more) instances where bolt bounce left the bolt not fully forward, resulting in a light strike.
- Two instances where there was a bolt out of battery detonation.
These issues and a cold and windy conditions on the range made me decide to pack it in early.
If any of you have run yours as issued, I can see how one might become soured on the CMMG units.
So, after getting the deficiencies corrected, I was able to come up with 14 different ammos to try in it.
The only ammo that would not cycle was Aguila subsonic. No surprise there as it won't cycle a 10/22 either.
In testing each ammo, I fired a single round to test for lock back, and then 5 rounds rapid fire for function and a second lock back. All tests were suppressed.
Ammo Tested:
Aguila Subsonic (would not cycle)
Aguila SV
Aguila Match Rifle
CCI SV
CCI Mini Mag Polymer Coated
CCI Blazer
CCI Clean Subsonic
Winchester Subsonic (Ran fine with a clean gun. Might be iffy when the gun is dirty)
Federal 510 HV
Winchester Wildcat
Remington Subsonic
Federal 750 Bulk Pack
Federal Auto Match
Winchester 555 Bulk Pack (36gr. bullet. Would sometimes not lock back)
Things that are nice to know if you are going to buy or build one of these:
Charging handle:
Spent cases get trapped between the charging handle and the RF BCG. Bore Buddy sells a charging handle insert, but at $10 I thought it was high for a plastic spacer. Turns out you need something as this happens with enough frequency to be a bloody nuisance. There is also a plastic RF charging handle, but being plastic it will break if you trip the bolt release and the handle isn't latched, or mortar it to clear a jam.
My fix was to run a bead of black RTV in the trough where the gas key normally rides. Then using my pinky finger, scrap out the excess as to leave about 1/3 of the trough filled with RTV. This is enough to prevent cases getting in there, but won't rub the RF BCG.
Bolt bounce.
Out of the box, the RF BCG weighs 4.75oz. and has bolt bounce visible to the naked eye when fired. This causes malfunctions in the form of light strikes when the bolt doesn't close all the way after the bounce, and shooting rapid fire if the hammer drops and strikes the bolt in mid bounce. It can also cause out of battery detonation and the resulting hazard of bits of brass case being blown out.
BoreBuddy sells a weight kit with changeable weights to adjust the weight to eliminate the bounce and tailor the weight to the ammo for best function, especially if running a M16 lower and you want the happy switch to work.
I added weight to bring the total up to the same weight as a 10/22 bolt as a starting point. This is what BoreBuddy recommends for subsonic and SV ammo. This worked well with all the ammo tested. No more bolt bounce, no more light strikes, no more blown out cases.
Increasing the weight further can be done when shooting High Velocity ammo to control RPM and the dB at the ejection port.
At this weight setting, some of the subs and SV ammo would occasionally fail to lock the bolt back on the last round, but it would still cycle. Ejection was also not as forceful.
The heaviest weights in the kit are recommended when shooting Hyper Velocity ammo.
Bolt buffer:
Included with the weight kit is a bolt buffer. Just like with a 10/22 where the steel pin in the receiver is replaced with a synthetic one, this little Delrin part cushions the metal on metal contact when the bolt cycles. It works as intended. However, it didn't play nice with the Redi-Mag adapter. It's thickness was just enough to prevent the bolt from locking open. I thinned it down to half thickness and all is well.
Redi-Mag:
This is an adapter to allow the use of S&W 15-22 magazines for ease of loading, excellent function, and last round bolt hold open. The adapter is inserted into the mag well and is retained by friction provided by a spring loaded ball detent. On my particular receiver, it was a very snug fit. Not so tight as to need a rubber hammer like superdrag, but it had to be finessed into place. This is no doubt the difference between a good and bad experience with this part. If it's loose, shim it with tape to make it snug.
There is a small steel piece that interfaces between the magazine follower and the bolt stop that must be installed correctly or the hold open feature does not work. And it is easy to install it the wrong way. The instructions that come with it could stand to be clearer and use a better photo of how it is installed. (See pics)
Once I got it installed properly and thinned the buffer, hold open works just like a standard CF AR.
Lubrication:
Users who comment on the net emphasize to run the RF BCG "wet" for best results. Previous experience with my other rimfire AR that runs a USGI M-261 unit is the opposite. Rimfires generate a lot of unburned powder and firing residue that sludges up the upper if a wet lube is used, so it has to be cleaned frequently, like every few hundred rounds. I've always used a dry spray lube on the M-261 and had good results. On this unit, I'm going to use Hornady's One Shot Gun Cleaner/Lube and see how it does.
For the CMMG Corp. Suggestion Box:
Add 1.5oz. to the BCG to mitigate bolt bounce and associated issues.
Furnish a charging handle insert to prevent empty case jams.
These two things will make it run right out of the box.
When viewed from the mag well side, this is how the little steel part should be oriented.
From the trigger side. Bolt stop retracted.
Bolt stop engaged.
Now for some suppressed fun! No louder than a staple gun.
