I was looking at a CMMG .22lr barrel so I can drop it on an A2 upper. Which is the correct bolt for this barrel?

If the cmmg barrel comes with a collar then any of the cmmg bolt/kits will work.
or you can use a Ciener or Spikes bolt as the collar that comes with the barrel will fit them too.
The best 22 kits are the cmmg stainless as they cycle the smoothest and clean easy.

What Redtazdog said I love my CMMG 22lr barrel it's the 16" M4 was going to cut it down to 10.5" to match my LMT upper but decided to keep it at 16.0" so my daughter could use it on her lower. I have both the standard bolt assy and the stainless to move from upper to upper. yes the stainless is easier to clean, just keep them lubed well and they are both easier to clean.the problem with the stainless being easier to clean is that the upper and lower are still a pain in the ass to clean and then re-lube. the lower it gets full of residue and it only takes about 500 rounds to gum shit up. still worth every penny!