I'm no doublegun aficionado but I've seen far worse with other types of guns that nevertheless possessed pristine bores. Guns used to come into the shop all the time that had been stored in a barn, pistols wrapped in a cloth, etc, where the outside was rough as a cob and the bores perfect.

Probably because the old-timer ran a greasy patch through it before leaning it against the grain bin and suffering the heart attack that caused it to remain there for the next 50 years...