You might want to talk to Rolf Baknick (spell?) who I believe worked as a smith at Century Arms.

Saunders also made (in JV with Wal Winfer) a number of large falling block action rifles in various cals to 600 NE. The remaining stocks of actions and barrels were sold off only a few years ago.

A few years back there was an article in a UK mag that featured someone who was building 4 bore double rifles and had options for matching smooth bore barrels. I think I might have a copy of that article somewhere.