I built the same kind of gun. New Frontier Armory AR45 upper, lower, BCG, and a Kaw Valley 8 1/2” barrel. The BCG is milled out for the ejector on one side but the face of the bolt on the other side has a sharp edge at the 7-8 o’clock position. Once that edge is removed the gun will cycle...
It’s the New Frontier Armory BCG. Load a brand new cartridge and let the gun jam. Remove the cartridge and feel the back of the case around the primer. You will find deep scratch marks caused by the bolt face. I had the exact same issue and I filed off the sharp edges of the bolt face.
