Probably the best method which is a tad hazardous is to plug the bore tightly and fill it with mercury.
Let stand for 5 or 10 minutes and remove the plug. The mercury will amalgamate witht he lead and when you remove the plug what residue that it left in the barrel is easily wiped out.
However, due to EPA regulations about mercury, it's not too feasible any more, although I can attest that it really worked quite well.
Probably the best method now it a good stiff (new) brush with some strands of a Chore Boy scouring pad wrapped around the brush. There is a lead solvent by I think, Shooter's Choice, but the smell absolutely nauseates me. I suppose you could plug the bore (and your nose) and fill it up and let it soak for a while, like maybe over night and then use the brush with the strands of scouring pad. Me? I just use the pad and brush. Even heavy leading comes out fairly easy with the scouring pad material and stiff brush. Light leading takes maybe 8 to 10 strokes to clean out, heavy leading maybe 20 to 30 strokes. At least that's how it's been in my guns.
Paul B.