SG, Leupold has a set of rings called "Rifleman" series, they come in low, medium, and high and are $20/set, you can order them from Gander Mountain and their shipping is always free. They are by far the best rings I have found for that kind of money. I mounted 2 scopes on 2 870's last year with them for a b-in-law and his son and they are nice rings. If you are using a saddle mount be careful not to overtighten the mounting screws through the receiver, the action will bind up if you overdo it, and a dab of medium strength LocTite on ALL THREADS. Not trying to be a know-it-all, just been through this stuff a hundred times. And good choice of slugs, Hornady SST's are super accurate and kill well!!
Yes, I do have a rifled slug barrel. As for slugs, Hornady SST is what I have been shooting out of the 12 gauge I have.
As for the scope, Im going witha 2-7x33 scope. Need to pickup scope rings now for it also.