A bullet drop compensator or BDC, is a device on a firearm scope that allows the shooter to adjust for the bullet's drop due to gravity. As the rifle is pointed upward or downward, the compensator adjusts the point of aim to keep the bullet on target. Nikon incorporates bullet drop compensators into their scopes to make them effective from a variety of ranges. Be sure to read our post on who makes nikon scopes to learn more about Nikon manufacturing!