Minecraft wiki entry for TNT_cannons, associated text: The cannon to the right uses dispensers to release TNT into the water. When you press the button, first the long row of dispensers will fire. After a 33-tick delay made the repeaters, leftmost dispenser will dispense the last moment before the TNT detonates, giving the shot a longer range. Note that the leftmost dispenser is higher than the other dispensers, and that the water should be flowing towards the front of the cannon. Dispensers dispense TNT without the characteristic "hop" of ignited TNT, which helps ensure the TNT will land in a much shorter radius than a conventional TNT cannon.