CVA used to make a single shot 12 ga shotgun years ago ithink it was called the trapper. I have not seen one since the mid 90's. Thompson center also used to make a muzzleoading 12 ga called the new englander. I have seen these for time to time on gunbroker.com, though they are pretty expensive and go fast. thompson center does make a muzzleoading shotgun barrell for the encore but this is definately NOT a old style shotgun. Several companies make reproduction flintlock fowlers and smoothbores. one name that comes to mind is caywood guns. these are kind of expensive though.
I wish a company would produce a economical single shot, caplock shotgun, or even if nef would offer a 12ga barrel for there sidekick muzzleoader. However, I have been wishing this for quite a while and it hasent happened yet.
Good luck
