I'm just wondering if that pitted barrel isn't adding to the patch 'destruction'. In my 54 cal Renegade I shoot 80 gr of FFg, wonder wad, and PRB with .015" Ox Yoke patches. And honestly, my groups are tighter than that at 50 yds. - a lot tighter. And my patches are more in line with the ones in Semi's post. You can almost reuse them.
With the accuracy you are getting I would seriously consider lapping that barrel or trying conical out of it. Its obvious that the PRBs in the barrels present condition aren't going to please you.