Check the crown. All my GPRs have had a very sharp crown which wants to cut patches on loading. The cuts mostly go away after 200-300 shots, but there's a quicker solution: Lay a piece of 320 or 400 grit emery cloth across the muzzle and press down firmly with the flat ball of your thumb. Rotate your thumb smoothly back and forth half a dozen times to break the sharp edge. Really quick and easy.
There's another possibility- Depends on how much you and the previous owner have shot it in total, but when newer the bores tend to be a little rough with sharp lands. That goes away and smooths out after the same 200-300 shots, but some guys prefer to lap. I've always just "shot through" the issue rather than lapped, so I can't advise you there.
But here's the deal: When my 54's have first started out they needed that .015 patch with .530 balls. But once I got through those 200-300 shots I started noticing that loading force got a lot lighter with occasional burned patches. A switch to .018 patches firmed up the seating force noticeably, but didn't quite require a mallet. Patches quit burning, and meanwhile accuracy got even better. After several K shots that combo starts feeling a little loose, but I haven't tried a possible next step cuzz accuracy has remained good. Next step in my mind would be to go to a .535 ball and .015 or perhaps even .018 patch, but I haven't tried that with either gun. Don't want to get into Mallet Land, and I'm still satisfied with accuracy.
BTW- Swapping back and forth between Pyro and Goex didn't change anything for me. Ignition after lots of shots is certainly better with the Goex, but you can beat that with a swab every half dozen shots or so when using Pyro.
Great gun, and it'll come around, whether you "shoot through" the issue with more shooting, or with lapping or crown polishing.