I am building a tennis ball cannon for my intro to engineering class at Arizona state. i wanted to know if anyone knew of any research or ideas about how large the volume of the air tank should be. i want to have enough volume behind it to make it effective but not have this overly large tank behind the barrel. right now the barrel is 2.5 inches in diameter and 2.5 feet long and i am using 3 inch pvc as the air tank. i know the tank should be wider but this is the best i can get.