When you are buying an air gun, airgun, pellet gun or rifle or even a bbgun, the manufacturers generally specify the velocity said gun will achieve. This is usually measured in feet per second or FPS, and may or may not have any resemblance to real world accuracy. Trouble is the manufacturers almost always have higher numbers than you are likely to achieve. They pick the best ammo or pellets or bb’s to measure the FPS and perhaps pick the best rifle out of many to base their measurements on.

Air gun enthusiasts often purchase chronographs or chronos to measure the real FPS of their air guns, using the pellets they use, and this way they can test various ammo to find the best for whatever type of shooting they do, be it target shooting by punching holes in paper, or shooting tin cans or hunting for small pests or going after larger game to put food on the table. More data is always better. It’s also very useful to have established FPS numbers for a given air gun and ammo, in case something goes wrong with the gun or the owner decides to improve on the tuning of the airgun.

Knowing the FPS of the air gun, and the weight in grains of the pellet or bb, you can find out the effective FPE or Foot Pounds of Energy that your air gun has. FPE can give you an idea of how hard a pellet is going to hit, and perhaps along with accuracy give you a good idea of how effective this air gun will be in a hunting situation. FPE might also be defined as knock down power.

If you have a Chrony and you now know the weight of your pellet and the FPS, use the calculator below to figure out you FPE or Foot Pounds of Energy or what I like to call the KDP – knock down power.