Sort of but not really an optics question. On an AR, am I better off mounting a 250-type kill light above the scope or off the quad-rail?

I put mine as far forward as I can and to the side, this seems to cut down on the rifle being able to reflect back into the scope.

I did this with both ir and green lights.

Take your pick. I have done it both ways with no issues. Just make sure there is nothing in front of the light beam to reflect back.

I like off to the side as well. First, it makes it easier to access if you aren't using a pressure switch. Second, it doesn't make your rifle awkwardly tall. Third, if you bang the light on something, you don't risk damaging the scope. Fourth, what Hopalong said.

Side....

I prefer to mount mine underneath the handguard/free float tube. It helps to keep the dust particles in the air from reflecting back into the scope and reducing the range at which you can see. It also keeps the light along the axis of your barrel rather than along side of it. I also don't like to mount it too far forward because of the muzzle blast. Offset to the side works well too but doesn't have the same range as underneath. Mounting it underneath will affect what you can use for a rest. So there is a ttadeoff. I suggest trying underneath and to the side and see which one you're most comfortable with.

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.

Look on Elusive Wildlife's webpage, they have an adjustable mount for piccatiny rails so that you can place the beam exactly where it needs to be. I would also place it on the side of the weapon.

Underneath.