Howdy!
By no stretch of the imagination am I an expert. I've never even seen a real (or repro) 1859 Sharps. Isn't that one of the guns that uses a paper cartridge that when the breech rises, it cuts the paper exposing the powder to the cap hole?
Well, let's look at this logically. I'm like you, Tangle Eye, I've always heard that airspace in a BP chamber is a recipe for disaster. So, I'd think that the paper "cartridge" holds the BP well enough to serve as a sort of chamber (that is full) even tho' there may be other parts of the chamber that DO have empty space. This, it seems to me, would be very different than actually having loose powder in the chamber that is only partially full. Now, a FULL chamber seems like any other BP fired armament, tho' I'm not sure how it would be compressed, which is ALSO what I've heard needs to be done for the optimum BP performance.
I wouldn't want to try loose powder (at least not a full chamber) in MY 1859 - if I had one, that is.