Wind

$pV=n$It is obvious what wind can do.  If you've ever driven on a Midwestern highway in the middle of winter, you may have been nearly pushed of the road by wind.  If wind can have such an effect on an object as massive as a car, it will definitely effect a bullet's horizontal trajectory.

Recalling a familiar equation of position from the Gravity section and modifying it for the z-direction, with the x-direction being parallel to the rifle and the y-direction being parallel to gravity, one could calculate how far left or right a bullet may stray off center.  However, this is very dependent on the properties of the bullet itself and it is easier to use manufacturer-provided values for given ranges and wind speeds.