Have you ever thought "I am shooting 400fps with 0.27g bbs, how many Joules (kinetic energy) my bb has?" or "If i used .20g bbs, how many FPS would I have?". Airsoft FPS is a program that can make calculations with a user friendly interface, relative to your bbs speed and weight. You can also enter the FPS limits of your local field and see if your bb gun complies with its rules.