Hand Lapping

Hand lapping casts a soft metal slug, the lap, directly inside the bore, charges it with fine abrasive, and works it back and forth on a rod by hand. As it travels, the lap polishes out tool marks and gently evens any tight or loose spots so the bore holds a consistent diameter from end to end. Makers verify that uniformity with air gauging before and after the work.

A smoother, more uniform bore gives propellant residue and copper less to grab, so a well-lapped barrel tends to collect fouling more slowly and clean up faster. The smoother surface also shortens the break-in period a new barrel needs to settle. Lapping must be done carefully, since removing too much steel can wash out the edges of the rifling.