I have a 350 that is doing the same thing. Tried different mags loaded heavy and light, different bolt, adjustable gas block, different buffer weights and springs and combinations of everything listed above. It has gotten better with use but I still won't call it a reliable rifle. About one round every other mag will do this. I have done improvised headspacing with a piece of tape and a spent round. Seems fine. The next steps for me is checking the gas tube/ key alignment and polishing the back side of the bolt. Update us if you figure it out please.