Nothing against Bushmaster; I owned two of them that functioned just fine (including an HBAR that was quite accurate), but in my opinion the materials and construction of the CMMG units are better. I don't know what was wrong with my first one, but based on their reputation I'll assume its an exception.
This is the rifle I've used the upper on:
-LMT A2 stocked lower
-LMT M16 BCG and charging handle
-TA31RCO-A4
-LT100 (one of the non mispec parts)
-RRA ambi selector (another non-milspec part)
CMMG govt spec upper including the proper barrel (profile, material, twist, chamber, and chrome lining are all correct) F marked FSB correct for the flat top. The only non-milspec feature on the upper that I'm aware of is the fact that it has M4 feed ramps, and I don't think the FN contract weapons do.
-KAC RAS
So, its not perfect, but its pretty close, and CMMG is, at this time, the only source for that upper that I'm aware of.