Has the problem been solved? I side with CW's suggestions. As someone else mentioned, a rim counterbore may help. I had mine cut and it shoots both rimmed and rimless well. Rimmed cases are easier to remove and load the rifle faster.
Hey Dinny. I just realized you were the first poster on this thread, and thought you might be interested to find out that a shim is what made that blackout barrel I bought from you work. The hinge pin had a serious head spacing issue, requiring a .005 shim. It's been running smoothly ever since I did the same job.
Actually, I just realized Dinny wasn't the first poster, he was the last. My mis-read.