I think the wild card is I've seen guns (Parkers) where the bores were mint and untouched and they still were struck thin enough to where the minimum barrel wall thickness was in the .020 category. They hadn't been messed with, that's just the way they came. Now in my one personal case the .020 was about 22" down the barrel (6" from the muzzle). I'd think that pressures would be pretty low by then. FWIW, I fired RST 16 gauge loads out of that gun.

foxes rule