Minecraft wiki entry for TNT_cannons, associated text: An arrow cannon technically does the same job as a dispenser just with greater range and area-of-effect. The general way to make these is to have a piston extended over where the mounting block on a normal cannon would be. This would be hooked to a repeater string exactly nine repeaters long, then inverted to keep the piston extended. The rest is simply the charge trough, wired up like a normal cannon. Then use a bow to deposit arrows on the piston head, load the charge trough, and press the button. What happens is: right before the TNT explodes, the piston retracts making the arrows fall. Then the TNT goes off, launching the arrows long distances. This cannon is commonly used in generally mid-range scenarios where damage to blocks is unwanted.