OOO buck might be a good comprimise on the penetration concerns. Is a rifled slug likely to go through an individual and still have any ballistic properties left or will it tumble around inside the person and therefore be of little risk to anyone else? I have never seen one used except on a paper target, but I would think they would not be likely to exit a person with much spin or force left in them. For me bird shot then a rifled slug in the second barrel, but I am wondering about the risk of using the slug? It is a 20 gauge coach gun.
"I'm what ya call a 'conscientious objector'; you know a coward".