A slightly rust damaged bore might actually be desireable. You might need to lapp the bore to remove sharp edges that will scrape paper or lead but likely not. I have a rifle with a lightly rusted bore which would not pass a patch undamaged. A little lapping and the patches get through intact. I think I just lapped it using Scotch Brite pads. Those are mildly abrasive. I'm now developing a patched boolit load for a rifle with a seriously rust damaged bore and they too can get through intact. I have now found a load that disintegrates the patch without leading the bore. That same bore could not handle a load that worked well in another rifle with a smooth bore.