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!
Any suggestions or thoughts on this?