On a related topic: Southern Ohio Gun has some Martini-Henry carbines in .303 British from the blackpowder era. The ad specifically states they are not safe to shoot with smokeless powder ammo; it's why they are selling them as "antiques" and not as firearms.
Therefore, if you wanted to shoot one, you'd have to make your own ammunition. I'm not sure if loading tables exist, but I'm sure if you searched the Internet you could scare up the specs on the original load and then work backward to find out how much Pyrodex or whatever to use in the cartridge.
I ask because as far as I know, no one is making blackpowder ammo for these rifles any more. Could you take modern .303 British brass and primers and the appropriate bullet, load the cases with blackpowder, and shoot them safely? Or would this be a dangerous thing to do?