could be you need to experiment with powder charges my .451 when I tried 70 gr of 777 2f would shoot about 2.5 in. at 55 yds, when I upped it to 80 gr. bullet holes were touching.. your gun may just need that perfect charge to shoot good... as I said earlier unless it looks like a log chain has been dragged thru it, it will more then likely shoot.. with some pitting you may have to swab every 2-3 shots as it will cause fouling to build up quicker.
I think taking it to a gunsmith will just be money thrown away as about all he will say is you will have to shoot it and see what it will do, you may have to try quiet a few powder bullet combos before you hit the right one, but I would say the .504 bull shops with what ever powder combo will be what shoots best.. all my Whites like bull shop bullets.. you might order some of Ed Mehligs 460 gr conicals also to try they are very accurate out of my Whites.