Got one 14.5" M4 upper direct from Bushmaster that was a nightmare. The barrel was torqued WAY too tight, probably at least 100ft-lb. It broke one bbl wrench trying to put a Knight's free-float tube on. I mean when the spec says to "triple torque" the barrel, it means to tighten it, break it loose, tighten it, break it loose, and tighten it a final time. I guess Bushmaster took it to mean tighten it to 3 times the torque value!!
The taper pins in the front sight tower were also way too tight for a brand new, never fired upper. Was way too difficult to remove them.
And finally, the crown of the barrel was a few thousandths out of alignment. Usually no big deal, but when mounting a suppressor, it can lead to baffle strikes. The suppressor manufacturer had to true the barrel before installing their adapter/flash hider.
I once remember a little bird telling me to stick with Colt. I wish I had listened!! Actually, I think Rock River is the next to try. From what I've seen they have the best product out there. The Colt M16's are absolutely the best, but their AR15's are now a far cry from the best!