Whatever the most open setting is, would likely be best. Unless it's REALLY old, slugs will be fine. The old timers, very often liked shooting slugs best through full chokes. The rifling on the rifled (or foster) slugs, is not to impart a spin, but to allow the slug to swage down to whatever the choke diameter is. The only chokes you would be unsafe to shoot slugs through, would be turkey chokes, as they are entirely too tight to safely allow slugs.
