Minecraft wiki entry for TNT_cannons, associated text: An automatic cannon can easily be made with this design. Substitute the button for redstone and hook the redstone up to a clock. Be careful not to set the clock under 4 seconds, the same time as it would take the TNT to explode. If the TNT is reloaded too quickly, the timing can cause the shot to detonate before the propellant does, destroying the entire cannon.