Does your bolt lock back? Can you manually rack and eject shells? If so, I would say that the problem is on the front end, not the back end. I assume you “cleaned” the hell out of “everything”? Did you check the piston itself to make sure there wasn’t any debris blocking the spiral cut on it? Likewise, chips/debris in the gas ports aren’t as easily spotted as you’d think. Run something through the ports to make sure nothing is clinging to the sides. If you still find nothing, keep shooting it. Break in is totally different for everyone. Let us know how it goes.