Tinker,
I have an 18 bore percussion double rifle (Joseph Lang) that I use with roundballs. I got a mold from Dixie Gun Works - they are cheap scissors molds made in any diameter in 0.005" increments. Frankly they work perfectly for me. I was amazed.
I cast pure lead bullets at a slightly hotter than normal temperature (about 800-825 degrees F).
If you measure the rifling twist and decide to try for a conical bullet check out
Mountain Molds . If you can figure out how to work the Online Bullet Design, you can have a mold made to any diameter you want. I've not done this yet, but I'm contemplating it.
Brent