Bob is spot-on with his suggestions. Round balls will likely be a failure, Ovny, especially for hunting. For really light loads with real black powder for plinking or shooting grouse at very close ranges, they might be ok.

The shallow button rifling on those rifles along with their fast (conical bullet) rifling twist is not meant for patched round balls. Bob's suggestion of the R.E.A.L. bullet mould is a good one also - I had thought they were heavier, maybe 250gr. and 300? The REAL bullet moulds I purchased and tried in my .45 calibre flintlock rifle, were 200 and 220gr.

As well, Hornady markets (or used to market) "Buffalo Bullets" in red plastic boxes. These are full size, dry lubricated bullets and are heavier yet and intended for the 28" to 32" twist rates of the inline rifles.

I actually tried them in my original .50/70 Sharps barrel and they shot fairly well - good enough for hunting deer or hogs to about 100meters - 4-5" groups at 100 with the carbine's original sights. Full sized bullets that I cast muself, shot best.