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.
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If your thinking about buying a new scope for your shotgun, you should look into one that is actually designed for a shotgun. These scopes have longer eye relief than a center firescope, and a parallax setting of 75 yards in lieu of a 100 yard setting onstandard scopes. I have a Nikon Prostaff 2-7x 32mm shotgun scope that works out very well on my Rem 870 slug gun.
I have a 3x-9x on ym slug gun and its never been above 5x for hunting. I just had an extra scope when i got the gun so it got put on there. If i had to get a new one i would get a 2x-7x.
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Another vote for the nikon pro staff 2-7x32. I have one on my 11-87 and it works awesome. I would also check into the new Nikon Sluggster. Its made specifically for shotguns with the BDC Reticle and is a 3-9X40.
2x-4x is plenty for me, even with the fast sabots.
All shots under 200.
The biggest thing you want in a slug gun scope is eye relief.
Follow up shots are really tough when your eyebrow is bleeding into your eye.
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