the barrel is correctly sized for the .22 bullet. .Certain brands of .22 leave a lot of residue in the barrel and even the gas tube. There is no gas tube on a.22 dedicated upper. Cleaning is a breeze.
I've got one of CMMG's dedicated .22 AR uppers and I'm very pleased with it. Like others have stated, it's nice not having to worry about cleaning is up for .223 usage, but the best part IMHO, is that you get the proper 1:16 twist rate that is optimum for .22 rather than trying to wring out some accuracy from your standard AR barrel.
Re-zeroed my irons for my drop in conversion, shooting CCI standard velocity at 50 yrds. Orange circle is 3/4 in diameter and hard to see at that range, but not bad.