Its pretty easy to have enough Kinetic energy to make them work properly. My friends bow shoots a little over a 400 grain carbon arrow including the 100 grain spitfire at about 250 fps and his spitfires have never failed to open. I shoot a about a 480 grain aluminum arrow including a 100 grain spitfire at about 240 fps I have never had a spitfire fail to open. I think aslong as you shoot a bow with a draw weight of atleast 40 lbs and a arrow and broad head combined weight of 350 grains should produce more then enough kinetic energy to 40 yards for the spitfire to open and operate properly.