Barrel Laps are used to very slightly enlarge and polish the inside of through holes. For knife making they are used to polish the inside of pivot holes in folding knives or to make adjustments in knives with pivot bushings.
Apply a small amount of a fine grit Silicon Carbide or Diamond lapping compound to the brass barrel during use. The abrasive grit gets embedded in the soft brass and will cut hardened steel.
You can increase the diameter of the brass barrel by tightening the set screw in the end of the barrel lap.