I have just built a large, what we call bluewater speargun which is to be used for hunting pelagic fish like tuna marlin and wahoo. For this kind of hunting you often shoot quite far (up to 25 feet+) and complete penetration of the fish is vital in order to deploy the special kind of tips we use to hold them to the line. The shaft is powered by rubber bands, in this case 5 of them so the propulsion energy is constant. So is the Drag coefficient, and we can assume that a thicker shaft will have a slightly lower ballistic coefficient. Most people use a 3/8 inch shaft for this kind of gun, where I have built mine to take a 5/16. 11/32 shaft are also available but from one company only, and in the US. (I live in Itlay) The shaft weights: 5/16 (8mm) - 700grams 11/32 (8.7mm) - 840 grams 3/8 (9.5mm) - 1000grams Here is the question: Given that the propulsion energy and drag coefficients remain the same, how much more kinetic energy will the heavier shaft take downrange? I am asking this question because I do not have access to any of the 11/32 or 3/8 shafts, so I cannot test it before I go modifying the guns' barrel channel to take a thicker shaft. I do however have a 9/32 (7mm shaft) that I could use to compare to the 8mm and hope that the differences might remain relatively constant between an 8mm and the 8.7mm. Should it hold true?? Anyone got an idea?