A lot of people put 3x-9x on their shotguns but go with a 40mm objective lens-the 50mm lens is so big it'll make ya mount the scope too high. I would reccomend a good fixed or lower power variable scope for it as a slug ain't going to be so long range that you'll need high power. A good 1.5x-4.5x or a 2x-7x orta be just right.