Have a look at 30-40 Krag balistics, you might be well to run a reamer in there, it would give you a bit of extra power to suit the weight of the rifle and is a low pressure round.

As it is somebody elses project I would unsolder the barrels and check the quality of the work, it is no biggie as if it was done right they will be well tinned and go back together easily, if not it is well to know now.

Good luck

Regards