I have a complete AR-10 upper minus the BCG .I then bought a DPMS complete BCG and the bolt will not enter the barrel extension .I tried it in every position so its not the extension being not turned to the right spot .I'm guessing either the extension isn't a DPMS or the bolt head . Any other ideas ? I'm going to take it to a WW 2 rifle match Saturday and a bunch of Tactical shooters will be there to set up the range after we are done so maybe I can get some hints from them . Arnie