I started casting and shooting shotgun slugs but they're limited as to the designs. What about staring with a somewhat oversized hollow base Foster slug mold and then running it through a swaging die to put the rifling fins and the finished details on? That way you're not squeezing all that lead out of the huge hollow base, etc, etc. It seems like you're starting with it semi-finished and then swaging in the parts you can't cast, like the fins, etc.
Ok, I know there's something obviously wrong with this whole idea otherwise something like this would be available.