A gun chronograph is an instrument used to measure the velocity of a projectile fired by a gun.
An early chronograph that measures velocity directly was built in 1804 by Grobert, a colonel in the French Army. This used a rapidly rotating axle with two disks mounted on it about 13 feet apart. The bullet was fired parallel to the axle, and the angular displacement of the holes in the two disks, together with the rotational speed of the axle, yielded the bullet velocity.
The modern chronograph consists of two sensing areas called chronograph screens, which contain optical sensors that detect the passage of the bullet. The bullet is fired so it passes through both screens, and the time it takes the bullet to travel the distance between the screens is measured electronically.