I picked up a scoped 98 Krag with a #4 condition bore as described. I think it was fired with corrosive ammo and put away uncleaned for decades. I used Hoppes #9, Sweets 7.62, Kroil, JB Bore Paste and Bronze wool on the bore. The bore is still dark and patches still come out with some black on them.

The rifle still shoots cast bullets in 5 shot groups of 1.5 inches or less at 100 yards. Amazing!