Got it. I had one of these in a Model 10 Predator Hunter in .223 with centerfeed dbm like yours. It would not reliably feed the long 75gr. AMAX etc from the mag. It was fine with a conventional HPBT such as the 69gr Nosler CC or Sierra. Was good to go with the various 55gr. pills also. I never fooled around with the feed lips as I only had one mag and didn't want to destroy it. You might try a different slug and see what happens.

I tend to agree with your assessment of the tool marks not having much to do with your feeding issue, even though it looks mighty unprofessional.