I used to run a Spikes .22 conversion in my AR's similar to the CMMG. I did not like the lack of accuracy you had, shooting a .22lr out of a 5.56 barrel. (my groups would end up looking like birdshot, haha) I ended up selling the conversion and going with a 15-22 and also a dedicated .22 upper (sbr) and couldn't be happier. Now I've got little tack drivers that eat .22 bulk packs like crazy. =)
I would highly suggest looking into a dedicated upper over a conversion.