There is no need to plug a barrel what so ever. When you take it out of the tank, pour out the water and wipe immediately. The whole thing dries instantly inside and out. Any discoloration in the barrel with come off with about two swipes of some JB on a tight patch. I've done dozens this way, rifles and shotguns with no rusting of the bore.
If you plug the plugs need to be perfectly sealed or you can get some water in the bore that will stay in there for the entire rusting/boiling routine.
Tom