In either case I am going to have to join the barrels with a rib. What method did you use? Having had a lot of bad experiences with old English double guns with barrel separation issues. I am wary of soft solder. Silver solder, and brazing, are nearly done at the same temperature and may be a real challenge with a heavy rifle barrel, being joined to a thin shotgun barrel.
At this point I am leaning towards the fixed barrels, with Siamese mule ear hammers, On a single plate, just so I don't have to deal with the rotational engineering involved in a swivel. I'm thinking the breech plate, tang, and trigger plate could be all one piece. The lock plate, and side plate, could fit together to form in essence a box lock. Does this sound feasible?
Hungry Horse