lapping
What some so called experts are calling lapping is actually cleaning. They are using JB Bore Cleaner, Flitz, or Simichrome on a patch and scrubbing the bore a couple hundred times. This is mearly cleaning the junk from the barrel.
Lapping is what King mentioned, using a lead lap that has actual lapping compound imbedded in its surface. It has to match the bore perfectly. Insert a couple patches to create a dam. A cleaning rod is inserted into the bore with an old worn out brush. Molten lead is poured into the bore to form a slug. Push the lug about halfway out of the bore and coat with abrasive compound and oil slurry. Run the slug back and forth in the bore. Remove the slug and thoroughly clean the bore with solvent to remove any abrasive particles. Repeat the entire above process with finer grit. You do need a new slug for each grit.
The problem with lapping is at the spot where the lap is stopped and the direction is changes, it has a tendancy to remove more material. Typically, barrel makers do this prior to fitting the barrel as they will cut off the last inch or more of the barrel where it is hogged out. The chamber will take care of the other end. On a blackpowder gun there is not chamber, but the muzzle end can get messed up.
If you decide to try either method (lapping or cleaning), always use a bore guide. You can have the most polished bore and smooth lands but if the crown is damaged, your accuracy is ruined. I use a rod guide every time a ramrod goes into the bore. Yes, I carry one in the woods for reloads.