Here is a good overview and low light primer for your AR-15 and tactical shotgun for the mounting and use of a tac flashlight.
Some great lessons out of this on the mounting of a tactical light. The biggest point is the more lumens the better and don't be sold on something different.
Another great takeaway is that you don't necessarily have to point the muzzle of the gun at people to illuminate the area and positively identify the person. This is a very important tip; you never want to point a firearm at someone unless you intend to use that firearm in self-defense.
In a low light situation with a quality tac light you will be able to identify non-threats (people you know) from threats (armed intruders) before taking up a point of aim.
The final point is, no matter what, train and train hard with your tactical mounted light under supervision and under dry and live fire conditions.
As we say in the military, "train as you fight" and become proficient in the use of your firearms in varying situations and conditions.