I have a Model 1863 Shiloh .54 cal I bought back in the late `70's. It was expensive then. I think I paid $225 for it as a used gun, which was a lot at the time. I've never been too clear on the right way to shoot or load it. I've always just dropped the bullet in, (which seems to stick in the rifling) filled up the rest with powder and closed it up. There's something of a hollow around the flash channel that obviously can't be filled with powder, and that always made me nervous about having "loosish" powder. Worried about barrel ringing is the issue.
I've never weighed the charge, or Chronographed the load either. So I don't know any specifics, other than it kicks pretty hard with a Maxi ball and FFFg. And I figured it'd be enough to kill a critter if I shot one. I hunted with it, but never killed a critter with it.
Would it be possible to make up paper cartridges that could just drop in there and have the block slice off the back end? It would seem to me that that's what they would have done back in the day, particularly for a combat application.
I've moved on to shooting more traditional ML'ers in the mean time, as, cleaning the Sharps involved a lot more disassembly to get at all the parts that got BP residue on them than a more traditional ML'er. I also imagine there is some gas leakage around where the block slides up too upon discharge. It's been at least 10 years since I last shot it though.