Modified slugs

That's a good idea. One thing I like better about the screw idea is that it's threaded on, so it's secure when cycling/being shot. I'm thinking if that BB were to rollout and get pinched between the slug and barrel, it could wreak havoc on the rifling.
One thing about something thread it does have a chance to go in cockeyed. I don't think it's worth tapping the h.p. first. Maybe if you go the bb or bearing route a little white glue to hold it in place instead of crimping it?