Sounds like a rifle chambered for one of the muzzle loading conversion cartridges like the various Roberts .58s in the US, Krnka .58s in Russia and several Enfield versions. See Hoyem's "History and Development of Small Arms Ammunition" vol 2 for some pictures and descriptions. One of the more common ones in Europe was the short case Snider which would fit your description pretty well.

With the complete lack of standardization in the early cartridge days the chamber will pretty much be unique to the gun. If you are lucky you might find some sort of currently manufactured brass that you can form to fit, otherwise you can send a good chamber cast to Dave at http://rockymountaincartridge.com/ and he can turn you some to match the chamber.

These cartridges were set up to contain a typical muzzle loading military charge - 50-60 grains of black and a 450-500 grain bullet. As to altering the chamber or using smokeless, I sure would not.

Sounds like a pretty cool early cartridge rifle, probably mid to late 1860s vintage. Pictures please!