Can you reach the barrel stuff with a drill bit, and drill a bit out to see what it looks like? Try to dissolve it with whatever?
Or take the breech plug out ( most of these old plugs come right out after an initial pop to loosen them) and see if you can tap the stuff out with a wood RR piece, from one end or the other. Maybe soak with WD40/thin oil for a couple days first.
John
