Combustible cartridge. Thin cardboard, like a firecracker, but it's doped with nitrates or something, so it burns up like the old flash paper. I suppose it would work with plain cardboard, but having it burn up would be nice. The bottom is crimped like a shotgun shell, and the top rolled down onto the bullet, with a bit of wax for waterproofing. Just like the picture, except I don't know a good way to show the crimp, and I'm not really good at drawing at this point anyway.
To fire, the hammer hits a piezoelectric crystal, which sends a jolt down the firing pin at the same time it's punching through the wax at the center of the crimp far enough for the spark to get to the powder. Powder goes off, out goes the bullet. Ideally you use the flash paper, so that you just open your cylinder and stuff in a few new rounds. The ignitor might be gotten from an electric cigarette lighter. Those use just this system, you depress a spring which pops up and hits a piezoelectric crystal, making enough electricity to create a spark. Black powder might need a bigger jolt, but ganging a few together might work.
These cartridges will probably not be strong enough for a semi, but will be made fairly easily. You might even later make combustible plastic cartridges that will be strong enough to cycle in a semi, instead of a punching in, the firing pin hits a spot in the back that's been doped with another chemical to make it a "primer" that will go off with the spark.