Eightbore,

Most of the guns that needed sleeving started life in the days of corrosive primers, which had particularly deleterious effect when nitro powders replaced black and boiling out became less common.

Internal lapping out should have only been carried out to remove pitting caused by the owners neglect.

Again, striking down the exterior of the barrels would only be needed if the outside had been allowed to rust more than superficially.