Saku:
Double-barrel bore-rifles like yours were regulated by inserting packing between the barrels somewhere slightly forward of the mid-point, to bow them out slightly, and thus create the correct amount of convergence of the bores at the muzzle. Same with SxS shotguns.

Tinker's right, just throw together some loads and get it shooting. It may take a while to tune the load for perfect results, but you can't do that unless you start burning some powder! Don't forget to use lubed wadding or a grease-cookie of some sort, as modern BP produces relatively dry fouling without it, which will damage the lead projectile on subsequent shots and ruin accuracy.

A round ball should probably be a few thou over groove diameter, which may not chamber when fitted in a plastic or paper hull if the gun was built for brass cases. In a brass-case double, you may need to crimp slightly, as recoil from the first barrel may inertia-pull the projectile from the case in the second barrel. Sounds like Tinker gets away with wax, though. Let us know how you go.

Tinker:
Any chance of some gobs of wax remaining stuck on the projectile and causing flyers?