An Airsoft Chronograph is designed to measure the speed of a projectile between two light gates. As they know the distance between these light gates the speed of the projectile can then be calculated - once you have the speed and then the weight of the projectile you can then work out its velocity. This is all calculated for you when you fire a BB through a chrono and it tells you your FPS (Feet Per Second). Most sites will have a site chrono so they can test the weapons of customers. Some players and Techs who are constantly tinkering with their RIFs might like to purchase their own Chronograph so they can test it after servicing or repair.