you can start with necked up 303 brass. it would be around 55 mm long but thats ok with black powder. of course 405 Win brass , dont think about because 9,3x74R is easier to get here. try this guy http://www.writerbobshell.com/
, he would have some die sets that can be used to form the brass till it fits into your chamber before you spend the money for a custum dié set. the brass have to be reamed out for 10 mm bullets and will be very thin after. good to have one who do it before.
I would start with 10mm /.40 pure lead bullets. the right weight for the 10x60R( if this rifle is chambered for this) was 16,1 gramm for a flat nose lead bullet and the rifle may be sighted for it.I dont see 10 mm moulds over 175 grain buts maybe a simple job for a craftsman to rework such a mould.
I dont believe in a "wildcat" cartridge, this double rifle was expensive and made for a man who dont need to reload his own ammo to save money. he had have enough money to shot factory made cartridges and in special it was a cartridge he can get everywhere not only in the springer's erben shop.
thinking it was the 10x60R 400