The originals were designed to use a primer, not a percussion cap. Primers were issued in tubes and inserted into a primer magazine that went down vertically in front of the hammer. When the hammer fell, cams on its inner surface activated a stud that kicked out the top primer so it passed over the nipple just as the hammer hit it. The system is rather amazing to watch, but probably not a lot of fun with original primers, which are now around $200 for a tube.
Unfortunately, the priming mechanism often fouled up and the fallback was to use musket caps, as is done on the repros.
The original cartridge was paper or linen; it was seated in the chamber and as the breechblock rose, a sharp edge sliced off the rear of the cartridge, exposing the powder to the primer flame. In most originals today, that edge has been eroded and rounded off so it won't cut cleanly.
But when the guns were new, they were pretty advanced for the time and allowed a good rate of fire, though they could not compete with the contemporary Spencer, which used metallic rimfire cartridges.
Jim