Overload, do you really want to be shooting something that close???
If your light system happens to co-witness with your impact area, then wonderful and use it, not all do, surefire makes great stuff, but my forend on the 870 is not in the middle of my pattern, at 10yrds, but at 10 feet I'm cool with that. Target it in 1st, and also remember things come loose, check the tension from time to time. Easy to just turn the light on and sight in to see if they all seem to match up as well, without actually shooting, at short distances.