Looking to build a dedicated .22 upper.   I have a 15-22 but it's not scratching the itch due to it's lighter weight and overall feel.

Looking at getting a CMMG conversion kit.   Does it matter which .22 barrel I use with this kit?  Planning on using CMMG mags.

I've assembled several AR's (6.8, 9mm, etc ..) is there anything drastically different about a .22?

Assuming the bolt assembly just recoils against the buffer since it doesn't have the force to move it?

Just buy a dedicated 22lr upper and be done. Much better than a conversion IMHO.

22lr carrier never leaves the upper during recoil and physically can't.

I used a CMMG bolt and barrel and just built a carbine. It is ridiculously accurate
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Using a CMMG conversion, the dedicated barrel, brand shouldn't matter just make sure you buy the barrel with a collar ( the collar  attaches to your CMMG conversion)
CMMG also makes collars which should work with most dedicated barrels.

The bolt can and will  move against the buffer, I use a pressure plug in my dedicated 22 AR's
https://taccom3g.com/product/pressure-plug-ar-22/

A Buffer and buffer spring isn't always necessary for 22 LR AR's example you can build an AR  22 pistol without buffer tube you just plug  the lower receiver https://www.brownells.com/rifle-parts/receiver-parts/buffer-tube-parts/buffer-tube-hardware/ar-15-m16-22lr-pistol-plug-prod54594.aspx

As for Mag's I don't like CMMG mags, I've had nothing but problem with them, maybe the ones being manufactured now are better ??
My go to mags are Black Dog mags
My second choice would be the S&W M&P 1522 mags but you'll need to use a mag adapter

I'd suggest using a  CMMG 22 LR charging handle or a filler for standard charging handle

I use a CMMG B collar conversion unit with a dedicated CMMG barrel for Steel Challenge....I have good results with Black Dog stainless feed lip magazines, and give them an occasional spritz of Hornady Cleaner and lube to lube the follower and follower track...a pressure type of plug is probably a good idea, I rebated the rim of my buffer so that it protrudes slightly forward of the detent and adds some pressure to the unit

if you are going to shoot any competition, would suggest the initial installation of a Bore Buddy firing pin and their wire EDM cut extractor... my  ~9" SBR works well with CCI SV and I'm shooting about 1200 rounds a month...

Just finished a build last week. I used a Beyer barrel, and bought the collar through them also. Phenomenal lightweight match barrel. The 16.5" bull barrel weighs less than the 10.5" barrel it replaced.
Beyer barrels

Alot of aftermarket single stage triggers will have issues with light primer strikes. The cmc 3.5 is reported to work well,  but I went with a hiperfire hipertouch reflex. Excellent 2.5 pound trigger that has 35% more hammer force than stock millspec.

Also installed borebuddy firing pin, extractor, pressure plug, and adjustable anti bounce weight kit.

Simple assembly once you have the parts..I just went with a CMMG Bolt and GP Barrel..All the rest on it is USGI that can be. Lower reciever is a M16A2 clone...Add some 50 BD round drums and a Franklin Trigger and you have a very fun setup..

I bought a dedicated upper from BCA and some Black Dog mags. Runs like a champ and shoots $.05 size groups at 25 yards with crappy ammo.

My kids & friends love it.

I just built an upper with this in it. Runs like an absolute champ and is pretty accurate (25y) with an Aimpoint PRO.