Outlaw 
I last summer bought a 12 gauge USH (SB2-980).  The scope I used was called the Nikon Slughunter, which has a BDC reticule.  The reticule has four lines, one being 50 yards, 100, 150, and 200.  Read the directions on how to site in.  It is a 250 dollar scope, but it is specifically designed for slug guns.  That is what I would get 
Hope this helps 
buckshoooter