[TheOtherGuy]

basically when the catapult is fully retracted, the cylinder sticks out 3 in. Before we shoot, we hold down the ball (from the top, we still have to make that part) and open the valve. Air from the tanks fills up that 3in. and we wait until it is time to fire. When we fire, we remove the top piece and the air inside the cylinders expands rapidly, launching the ball.

Maybe I should make a quick diagram...