I saw on The Yogscast's channel a cannon that auto reloaded, so I decided I wanted to try and figure out how to make it. So here it is.

Additional Notes

Just fill it up with the TNT when its finished, save 1 space oposite the buttons with water, and fill sand up 12 blocks high from the last tnt line. Don't fire too quickly or it will skipp steps and just blow it all up.