Minecraft wiki entry for t-gun, associated text: Make a normal cannon barrel, then break the blocks to the sides of your mounting block. This is where the aim TNT will be. Then, place water 2 blocks to the sides from your main "barrel". You can prevent it from pouring everywhere by placing blocks (e.g. slabs as in the schematics) around these troughs. But one block must be an opaque block, as it will hold redstone to power a piece of aim TNT. Also, the water must flow towards the main trench.