Due to differences in barrel machining, muzzle breaks, propellants and other factors, when the same bullet is fired from different rifles there will be a slight difference in the drag the bullet will experience. The Hornady 4DOF solver assumes the bullet will be fired from an "average" barrel but in reality your rifle may impact the bullet slightly differently. To account for any differences the Axial Form Factor value can be modified to align the output from the solver to what your bullet is experiencing. By firing at a known target and adjusting the Axial Form Factor value till the predicted solution matches real world results, you can calibrate the solver to match your rifle and bullet and deliver even more accurate firing solutions. Axial Form Factor can be calibrated using a target anywhere from 300 to 800 yards away and impacts the solution through the supersonic, transonic, and subsonic flight of the bullet.