What Dan said.
After receiving a long rifle that Taylor built, from a friend who passed away, we found the bore to be pitted end to end. Peter always used boiling hot water,w hich caused flash rusting- every time he cleaned the gun, which happened even before 'tea' after the shooting. ended. Peter always used real black powder, never a fake, so the pits were an accumulation of all the flash rustings - his bore never rusted in storage.
The fine pits held fouling and were nasty to load over, so Taylor lapped it with a lead lapp. IT was improved considerably, and another lapping finished the job- probably a total of 10 hours work. The gun shoots find now, with barely any 'fouling' hanging on.