When using a rifle, we have something at
close range called "sight off-set".
This is the amount that a bullet will strike
below point of aim.
With an AR15, the sight off-set at 10 to
15 feet (the width of a room) is about 2
This is caused by the height of the sights
above the bore.
You must aim approximately at the top of
the forehead to strike in the proper zone.