Chronograph

From Airsoft Community Wiki

A chronograph is a device used to measure time to a high degree of accuracy. In airsoft, a specialised form of chronograph calculates the speed of a projectile by measuring the time it takes for the BB to pass between two points, and gives a result commonly expressed as FPS (feet per second). Measurements are taken using a standard weight of BB (0.20g), and a calculation can be done using the weight and speed to derive a value for the power of a gun in Joules.