Diggory, very good explanation, but for you there, in order to sell a gun it has to pass proof. Here there is no such thing, and in my opinion if a seller sells a gun with known bores of .020 or less (seven sheets of standard printer paper), especially within the first 17" of breech, that is unjustifiable and shows he/they are only interested in the money and not your safe being.
Here we have a variety of people, and hopefully the ones buying these old vintage doubles have some common sense in the type of shells they use in them, but there are always some that don't.
The old saying Caveat Emptor should not apply to guns with less than minimum wall thickness, and sellers should realize this.
Just my opinion.
