I did have a single barrel 12 bore with Damascus barrels and originally black powder proof. It started life as a 16 bore and was then later bored out and re-chambered to 12 bore and then nitro proof tested.
These singles were often made with thicker barrels to tame recoil and this just goes to show how much can be taken out and it still pass as safe. It did kick a bit though. Lagopus.....