Not an expert by any sense, but it looks to me like there's a lot of hot gasses blowing by the patched ball to shred them like that! I'd guess the barrel might be pretty rough inside. I'd try a heavy-duty cleaning with plenty of brushing, then lapping it. When that is done, try the PRB before you shoot any conicals or sabots through it. The sabots or conicals could be leaving a lot of residue in the bore if it is rough. Patch quality & lube quality could contribute to the problem, but with a good barrel it usually isn't that critical.
Not recommending it - but I wonder what would happen if you used an abrasive (toothpaste or grinding compound)as patch lube for a few shots??????