Halo's are the undisputed kings of the reflex sights, and for good reason. There is nothing better for fast shots! But my EOtech 552 is too bulky for shotgun work IMO. But there is a whole new generation of smaller and lighter halo sights out there from EOtech, burris, bushnell, and others.
As far as mounts, I would use a rail myself. It'll allow more flexibility where you put put that thingy.....
