You can only go so far with the history of the American doubles their collapse as the repeaters appeared was complete. However, the English trade has a long and rich history of basically making the same 'form' of firearm, the double barrel. We could also apply this discussion to the French, Belgium, Italians and Spanish gun industry.
I'm reading through my new book about Thomas Horsley that I recieved the other day, looking for some information concerning the Scotish trade as well!!!
Great Book by the way!!!