Back in the late 70's a friend of mine got a letter back from England, straight from Holland & Holland (not the NEW-ISH US barrel company).
Will had sent a 6 bore ball and shot gun, a SxS cap lock with 36" tubes for re-furbishing and to have a case made. H&H re-bored the gun to 5 bore and sent a letter back telling Will how to clean ALL of his antique SxS's used with black powder, rifles and the shotguns.
They instructed - use tap water - cold tap. Do not EVER use hot water due to the flash rusting caused by a very hot barrel drying too quickly. Also, if there is built up fouling in the breech area, hot water can 'glaze' the surface of the fouling, instead of dissolving it. They instructed to dry the bore with patches, flannel best, then to use a water displacing lubricant flush, like WD40 to CHASE the remaining moisture, if there in the breech, from the barrel. Flush does not mean a damp patch. Flush means flush. If the WD40 does not blast from the nipple seat or vent, it is not flushing anything. If this seems extravagant, buy it by the gallon can - lasts a long time (over a year of weekly shooting) for the price of about 3 regular aerosol cans. A plastic spritz bottle comes in handy from the Dollar Store.
Cold water - 1 doubled patch to clean, 4 or 5 doubled flannel patches to dry - flush - wipe down, re-assemble and finished in 10-15 minutes. Takes over 30 minutes (sometimes over an hour) to properly clean a modern rifle's barrel depending on the copper fouling.