IIRC, for a while someone made a shotgun slug that was steel, sort of looked like an elongiated 8, and had lead driving bands. They had photos of it penetrating something like 55 Chevy bumpers. That would solve the breakup/penetration problem and with modern powders you could probably push it up to about 1400 fps.
Or just look at the shotgun from hell thread on several forums. They are turning a shotguns into a .729 cal rifle.
