ok so i was thinking a bit and the RSS look just about right for this.
you could use 1 or 2 lead round balls of .500 cal will be about 188gr each in pure lead.
or 1 .500" hardened steel ball for some nasty penetration.
of corse this would be for smooth bore guns and the steel one sould only be used
in a smoothbore with cylinder bore.
the lead one could be used in a muzzleloading shotguns too with 1 or 2 balls
with the sabot on top of card wads for it never touch's the the plastic.