I have a new 20" upper built to the USMC SAM-R specs as close as possible. I took the upper out this afternoon. I had some major problems with it running. I initially tried to use some white box Winchester 55gr ammo to get on the target before I went to the BH 77gr stuff. I had almost 100% failure to cycle,feed, extract, you name it it didn't work. I then went to some IMI 62gr thinking that the 55gr just wasn't hot enough to cycle the action. It functioned 100%, as did the BH 77gr. The Winchester however ran like a top thru a stock A2 rifle as well as a 16" carbine. Loading one round using all three types of ammo locked the bolt back 100% of the time. I tried the 55gr stuff with several different mags also, all USGI (both 20 and 30 round)without any significant changes. Two times I had to take the upper and lower completely apart to get the bolt back off of a live round that was partially chambered.
Any suggestions or thoughts on this?
Edited to add, Bushy lower, Bushy flat top upper, new Bushy bolt and carrier.....
I have had problems such as yours using NEW barrels. Sometimes they seem to 'prefer' a certain ammo over others till they get broke-in real good.
The only Ar's I have owned that would consistently shoot 'anything' were the older Colts in the 20" configuration. Nearly all of the Bushmaster brand rifles that I have looked at were 'much tighter' than Colt rifles at the same store... My guess would be that your gun is just new and tight and needs to be shot a bunch to help it loosen up a little...