I agree with this regardless of what the gunsmith said. You gotta have a burr of some kind in the chamber. Normal handloading problems doesn't produce this kind of thing. I seriously doubt this will be solved with changing primers and powders. I hate to say it, but I think you should try another gunsmith.That is a burr in the rear of the chamber. Probably rolled a chip while cutting the chamber. Probably just need to be polished out. Bring the cases with the scratches with you to show the gunsmith. Simple job to polish it out. Shouldn’t take long at all.