I have a RRA that is a SBR. I love it. No problems at all. RRA is come out with a fordged lower rec. for it. You might want to check that out. Should stop some of the problems with the inserts. Good luck.
Bud...do a search over on m4carbine forum...the carbon Bushy's aren't real popular if I remember right...I don't remember the reasons, its been a few weeks since I read that...but theres some info over there you may find helpful.