You really wanna get one that is close to flush with the barrel because you don't want to cast big shadows. Hmmm do I aim my light or my shotgun?!?
I'm sort of thinking optic rail mount, up top on a tactical 870. That way there's a shadow to be sure, but it's all below the barrel. Full field of lighted view across a scene. If a target is just below the shadow line it's probably being flagged. If it's above, I'll be able to see it clearly with no shadows coming from the gun. Of course it could also be mounted in a bit of pic rail just near the muzzle.