KCrow,

Welcome aboard!

You have a nice and very interesting double rifle there. Has Boss identified what cartridge it is chambered for? You definitely will want to start out with chamber casts before doing anything. Get some Cerrosafe from Brownells and cast the chambers along with the first couple inches of each barrel. You will get all relevant dimensions from these casts, and if you need custom dies RCBS will use the casts to establish dimensions.

Once you know what cartridge it is or what case to use for brass, and once you have dies that are suitable, it should be fairly easy to work out a load.

But again, your first step will be casting the chambers and leade.

Good luck,
Curl