Ranging Sights
Adjusting the sights can allow a player to engage enemies at a distance more effectively. Adjusting the sight ranging or zero means that the player's point of aim represents where the bullet will land at the specified range.
For instance: the M16 is zeroed for 250m by default. That means at 250m the bullet will land at the point of aim. Since the weapon has a high muzzle velocity the bullet trajectory will be very flat from 0m to 250m, thus the player does not need to adjust their aim for distances under 250m.