Rifled barrel - Sabots or Hybrids (hybrids are slugs that have a wad of sorts that flies downrange with the slug, acting like fletching on an arrow to stabilize the flight. Brenneke and Remington buckhammers are examples).

Smoothbore - Rifled slugs and Hybrids.

If you shoot rifled slugs through a rifled bore you will get all kinds of lead in the rifling grooves, and it is a pain to remove.