I use mine mostly on a couple of 22 rifles that are lever guns with a hooded front sight. The light that bounces back off the front sight makes it almost impossible to use if I mount it above the scope. What works best is to mount the light on the bottom of the barrel. The feed tube makes that a less than perfect arrangement, but I found that if I wrapped the barrel in a strip of soft leather (oiled), the mount is secure. And the light isn't in the way like it is on top or on the side of the scope.
If I had an AR type rifle, I would definitely mount it on the side rail.