When you reach Zone 2, you will be introduced to the Distance feature. This feature makes it so you have to alter your aim on the target depending on how far away they are. The distance of the target is indicated at the bottom of the screen. You should also see a range of numbers underneath the reticle when you zoom in. You will need to align the target to the correct range using the numbers underneath the reticle as your guide.
Note that this feature is only available in Campaign and certain PvP missions.