One of the last steps a custom barrel-maker does before shipping a tube, or installing it on a rifle, is to hand-lap the bore. Lapping uses a lead slug cast to the bore, impregnated with abrasive and worked back and forth through the newly crafted barrel. The lapping removes miniscule tool marks from the bore with the intent of reducing fouling and improving accuracy.
High-volume production barrels do not get lapped. Most production barrels are more than good enough for their intended use, and the time and cost of hand-lapping is something most deer hunters wouldn't pay for.
Barrels that haven't been lapped can sometimes be improved by lapping, but few other than barrel-makers have the proper tools for the job. Fire-lapping a bore, by shooting bullets impregnated with very fine abrasives, has been a subject of experimentation for some years now. The trick is always to get the proper amount of the correct grade of abrasive evenly distributed on a bullet.