As a bullet passes through the bore, it will smooth or burnish away these surface imperfections. However, as the imperfections are smoothed down, small amounts of bullet-jacket material can become trapped beneath them leading to increased metal fouling. By initially cleaning between each shot and then between each group of two, three, five or more shots until the process is complete, the break-in process produces a smooth bore with no lead or jacket fouling embedded in the metal. This produces a barrel that will not foul as quickly between cleanings, and that can be cleaned more easily. Barrel break-in need be performed only on rifled bores.
There are several different procedures for breaking in a new barrel, although most of them use the same basic process. One popular method is to thoroughly clean the barrel between each of the first 15 to 25 shots, then between every two or three shots for the next 10 shots. Finally, several five- or 10-shot strings are fired with a complete cleaning between each. Barrel break-in is typically completed within 50 or fewer rounds and is usually signaled by a noticeable reduction in fouling during cleaning.