Not sure on the cmmg's, but the Ceiner's always had a break in period. I took a dremel with a polishing disc to mine, no problems since. currently running a dedicated 5.5" upper
If its anything like the cieners (which its a direct copy of) it probably will not like the Remington Golden Bullets in the bulk pack. (green box) My kits like the federal value pak from walmart, and also run great on the federal auto match and the winchester dynapoint, remington Thunderbolt, and just about every other cheap ammo I have put through them other than the G.B..
They seem to run good on most of the quality ammo, such as mini mags, stingers, etc... but the cost of those add up quick. If you have a new one and it is giving you a few hang-ups, I'd suggest running a box or 2 of the mini mags in there to help smooth it out.
federal 550 in the maroonish color box works really well. best so far for me. might be a good idea to lube the rails with a nice thick lube oil like I stated earlier.
Just curious is there any kind of bad effect on the barrel from shooting .22's through it? Or is there any kind of special cleaning that needs to be done?
I did read the "shooting a couple .223 rounds through after firing idea"
Thanx