A chronograph is a device for measuring the speed of a bb pellets fired from an airsoft gun or bb gun. Normally airsoft skirmish sites will need a true measure of how powerful a gun is shooting so it does not hurt anyone too much when they are hit, the power rating is measured in fps (feet per second) Airsoft chrono's can also measure how many rounds per minute or per second an automatic airsoft gun is shooting. The muzzle velocity or fps is dependant on the weight of the bb pellet most of the time airsoft guns are chronoed with 0.20g bb pellets to see if the gun is within the site safety limits. a 0.12g bb pellet will travel faster than a 0.30g pellet fired from the same airsoft gun but the force of impact will be exactly the same this is rated in Joules. most sites will have a 350 fps max for an automatic weapon this is about 1.14 joules (just over 100mph) with a 0.20g bb.