with the things we are knowing now I would say its an older blackpowder cartridge similar to 45/70 loaded with BP and a bullet not heavier than 300 grain. a possible cartridge could be the 10x60R 400 if the chamber is long enough. this is a shorter version of the 400 BPE 3" Purdey and was popular around the turn of the century in austria.
BP cartridges may affect the value as a hunting rifle, practical and economical in the real world. in any case you have to load your ammo with a custom die set and make your own brass. but its a nice double rifle for sure.
a chamber cast will show it

its hard to judge such a double rifle because they are allways rare and unique. in germany it will be go in the 2000 -3000 euro range in an auction

Last edited by pwm; 06/17/11 11:28 PM.