I cast my own 12 ga slugs. As the attendees at the Texas Meet and Greet will attest, I get some pretty good accuracy (trust me you do not want me shooting at you inside 300 yards). A 7/8ths oz Foster slug (hollow base) at 1580fps is nothing to sneeze at.
Rifling on slugs does nothing to spin/stabilize. The "rifling" is there to allow the soft lead to swage down to whatever choke constriction the particular gun has and keep the pressures down. The stability comes from the hollow base/nose heavy design.