# Calculating Recoil Energy

`Q: In earlier issue you answered a question about free recoil energy between a light, fast bullet and a heavy slow one where both had the same muzzle energy. Could you run the calculation again using the IPSC Power Factor Formula of bullet weight x velocity/1,000?Johnno JohnsonA: I think it would be a lot easier to understand if I showed you the calculation for free recoil energy. Since the power factor formula is just a variation of momentum (mass x velocity) which is also a component of recoil calculations, there is an indirect relationship. If the power factor is higher, free recoil will also be higher, but I'm not aware that any other valid comparison can be drawn. The calculation of recoil energy actually has three parts. This is: I=(WBxVB +4,000xWC) 225,400 "WB" is the weight of the bullet in grains; "VB" is the velocity of the bullet in FPS; "WC" is the weight of the powder charge in grains; 4,000 is the assumed velocity of the powder gas: 225,4000 is a constant to convert units to allow the use of grains. The second calculation is for the velocity of the recoiling gun VG): VG=32.2xI/WG. "WG" is the weight of the gun in pounds; 32.2 is the acceleration of gravity.  The final calculation gives you the free recoil energy of the gun (EG) in foot/lbs. These equations can be found in Understanding Firearm Ballistics by Robert A. Rinker which offers a more complete explanation than I can go into here. I am not sure of what you want to accomplish by comparing power factor with free recoil energy. There is a lot longer discussion of recoil in Hatcher's Notebook which is also valuable reference.`

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