One cannot react to a shot at the speed of light, one doesn't know the shot has happened until it hits, or doesn't. Furthermore, there is a big problem when it comes to light, the one-way speed in one direction or the other is undefined (and not simply unknown), unless one can define what is "the same time" in two different locations. This really matters once we start talking about objects moving over ~10,000 km/s due to relativity. After we can decide that "simple thing" we now have to look at Lorentz transformation, which is dependent upon the local spacetime reference frame and the gravitational interaction upon said frame.