Minecraft wiki entry for TNT_cannons, associated text: One more thing you can add is this piston. It will pull away the down-facing charge piston so that it isn't in the way when it's firing. And… that about wraps this tutorial up! Experiment with the cannon and test to see what mounts fit your needs. Make sure to pull the ammunition out of the magazine before you test blindly—I can imagine it wouldn't be much fun to watch something that took this long to build fly to pieces!