This isn't a homework question; however, it is probably similar to one. I am doing a design project that involves a hail cannon. Basically I have created an air gun that propels ice balls. I have a compressed air tank, with an electronic quick release valve and then a barrel made of PVC. My question is this, I have tested the gun with different size hail. I have a chronograph to measure velocity, and I have a pressure gauge in the air tank. We tested different air pressures until we found one that resulted in a particular velocity. (In this case its the terminal velocity of falling hail.) Anyway, I now have to calculate what these pressures should be theoretically. Ive tried using Bernoulli's equation with little luck. I get answers but the theoretical pressures come out higher than the empirical pressures. Obvsiously this is impossible, so either I'm doing it wrong or I'm not seeing the over all picture. Can anyone give me some suggestions on what equation I can use or maybe where to start. For the record, empirically we have 6.5psi shoots .5" diameter hail at a velocity we want. When doing it theoretically, the same diameter hail shoots the same velocity at 9.5 psi. Hence the reason why I think im going wrong somewhere. Any help is appreciated.