Fire Lapping
Firing abrasive-coated bullets to polish a rough bore; effective at smoothing tool marks but easy to overdo, so it is used sparingly.
Fire lapping uses bullets coated in a fine abrasive compound that are loaded over reduced charges and fired through the bore, so the projectile carries the abrasive down the rifling and polishes out tool marks and rough spots as it goes. The aim is a smoother surface that fouls less and cleans more easily, especially in a barrel that copper-fouls badly from the factory. It is the shooting-based cousin of mechanical hand lapping, which a gunsmith does with a lead slug on a rod.
The catch is that fire lapping removes metal every time you do it, and overdoing it can open the throat, flare the muzzle, or wear out an otherwise good barrel. For that reason it is used sparingly, with a small number of rounds and careful inspection between them, and many shooters reserve it for barrels that genuinely have a rough-bore problem rather than as a routine step. Some treat it as an alternative to a traditional barrel break-in, though it is more aggressive and harder to reverse.