I too am in the process of setting up a shotgun with a rifled barrel. I just bought a Bushnell Elite 3200 in Firefly and got bit with a heavier load trying it for the first time. Right now my thinking is at least 4" eye relief, short length-nothing longer than 11" to clear the rear sight and leave space for a flip up cap and a heavy recticle. A Leupold or Burris seem to be the best options from what I have found so far.