Chronograph

Chronograph

A device that measures projectile speed.
Chronographs are used on most, if not all airsoft skirmish sites in order to check the speed at which the BB is shot from the gun. If the BB is travelling too fast, the gun will be considered unsafe for open play and its use will be disallowed.
Chronographs generally work by sensing breaks in the available light as a BB passes over two in-line sensors. The time taken for the BB to pass the first sensor and reach the second sensor is used to calculate the over all speed (Muzzle Velocity) of the BB. The time measurement used by Chronographs is in Feet Per Second or FPS.