kuduae,

Thanks for that excellent info! How many guns can be dated from a single proof mark! - not many I'm sure. Even if the maker is still in business it can be hard to date a firearm.

I did drop a .463 bullet down the 47 bore barrel but found it would stick half way down the 48 bore barrel. However it only just stuck, so the bores are still very close in size.

Do you know if the cartridge naming convention at the time would relate to the bore size? Should I call this a 11.7mm cartridge? Would the bullet have been bore diameter like som English cartridges?

Steve