Doug,
You'll need have someone make a chamber cast and 'slug' the barrel. With that accomplished, you'll know your groove diameter and case length. The former is probably .510 and the latter 3" or 3 1/4". A slightly compressed charge of Pyrodex or 2F black powder with a 440 grain Barnes Original bullet will be a good starting point - if you have the equipment and skills required. Loading large British straight-wall cases can involve some 'advanced' reloading skills. If you are intent on loading them yourself, the 'bible' is Graeme Wright's book Shooting the British Double Rifle. It is a superb book.
You should consider buying loaded cartridges from Superior in Sturgis, SD. It's pricey but you aren't likely to shoot it much and it might require less of an investment than buying the equipment and components necessary to load them yourself.
