Well, its a black powder gun, in that the cartridge was originally loaded with black powder. It uses the same cartridge as the Remington double derringer. Fresh .41rf ammunition is US$55/50 at Dixie Gun works (which does no one here any good). One way of preparing ammunition for it is to use machined brass cases which are bored off-centre for .22rf cases. The .22 acts as the primer. It has been reported that .41 Colt cases can be shortened, and bored for the .22 case. Put it this way: it is possible to get ammunition for one of these, but there is more effort required than with most other old ones.