I like the subsequent purchaser argument put forth previously. I once owned a hammer gun proved as a 12 with 2 3/4 chambers and then successively enlarged to 10 bore with 2 7/8 chambers, presumably by various generations of American gunsmiths. The guy I bought it from was insistent it was a 10 and did not believe it had started life as a 12. When I sold it, it was only with full disclosure of the modifications.