Appears to me that brass has two opposing burnish marks on the outside rim. Neither looks like
ejector cutout. The primers look "unremarkable" (as doctors say)
Instead, the burnish marks appear consistent with a possible uneven bolt face that scrapes the case (which has
stretched back to meet it on firing) upon opening rotation.
- Are you able to confirm that the bolt face is
totally flat?
- (Since you build ARs) do you have another bolt that would give you acceptable headspace to try out?