Get some good cleaner like 409 or Simple Green and a Scotch Brite scrub pad. Spray the breech plug down and then use that Scotch Brite on it. Normally that takes all that hard to remove fowling off. Also a good solvent like Birchwood Casey #77 will also remove that. Butch's Bore Shine for Black Powder is another good solvent. Rusty Duck Black Off (sold at Wal Mart) is another good one. All of them will clean that hard to remove fowling.