As I understand it a bore proved and marked 12,as opposed to 12/1 or 13/1 would have taken a .729 plug and not a .740 plug when gauged.
Therefore actual bore diameter could be anything from .729 to .739 from new.
It seems very common for older black powder hammer guns to be proved as 13 or 13/1 bores.
I even had one that was a 14 bore .
As has been mentioned previously it is virtually impossible to burst a normal barrel much forward of the chamber from pressure alone.
In order to do that the pressure at the breech would have been such that it or the action would give way first.
However Burrard does point out that very thin barrels can give way from normal pressures but he was talking paper thin or heavily pitted in a concentrated area.
Interestingly he goes on to say such bursts are notable for their lack of violence,said barrels just peeling open.
Not that I would particularly want my hand there at the time,violent or not.
Personally I would quite happily shoot a sound barrel honed out of proof so long as the chamber was original and free from pitting and the wall thickness was fairly normal.
Besides this is all you can do with an American made gun is it not?
