i don't know bout cmmg 9mm, i have 1 of their uppers in 5.56 & works great.
my 9mm is a complete rock river set up, but with either cmmg or rock river i don't think u can go wrong.
i bought a complete upper from RR & a stripped lower & put what i wanted on it & this is the end result.
all that i'm waiting on now to finish it up is my can then it'll be complete. then i think i'm gonna put together a sbr in 9mm as well.