RE: which rifle slug?
Slugs are expensive! I would go out and get a laser boresighter (something that you will be able to use many times over the years) and get your scope dead on boresighted and then double check the scope by firing some cheap slugs at 20 yards. THEN and only THEN would I move out to 50 yards with the more expensive slugs. I like the Remington Core-Lokt, and Federal Expander with the Barnes slug.