Concur with Der Ami completely, if you get the gun you will start your odyssey with a chamber cast and slugging the bore to determine what cartridge and bore size you actually have. Once you figure that out you will then begin the search for loading data if it exist, what case will work best, then get dies (CH4D is a good source), probably buy quick load software to help develop loads, and it goes on.
Listen to Der Ami, he guided me through a similar adventure with a Springer 10.75x52R double rifle I came by with great results.
One more thing if you can find it is the book “Shooting the British Double Rifle” by Graeme Wright, it provides invaluable insights into understanding the loading and shooting Double Rifles.
Last edited by old colonel; 12/20/22 09:52 PM. Reason: Added