This topic has been brought up before, just search a bit for it. We built an air cannon robot
last year and so did Team 1147, they brought theirs to a Kings game if I remember correctly. Ours uses a 4" barrel and can launch just about anything. We run it off of two large tanks as shown in the picks and bring them up to 100 psi with a separate compressor. We then actuate the gun using a spike relay controlling a sprinkler valve solenoid. We usually get 6 or shots before we need to recharge the tanks.