rigbymauser:
These photos of original fully-rifled bore-gun projectiles may be of some use:

8-Bore

12-Bore:

Photos of the Fosbery (Paradox-type) projectiles are more common because they were considered more interesting than the normal conical ball. These conical ball cartridge photos are from George Hoyem, Vol.3. I'd recommend finding a copy of that fine publication if you are serious about bore-gun chamberings, and British sporting ammunition in general.

szihn is quite right: it is just a conical slug, with shallow lube grooves. Cut your mold to give a projectile a thou or two over groove-diameter, in the correct weight range, of the approximate shape shown above. Simple!

Hope this is of some help.