Minecraft wiki entry for t-gun, associated text: On another side of barrel, place repeaters until you'll be 2 blocks away from a piece of redstone that will power the other aim TNT. These repeaters must all be on 4 ticks. Place a block in the front of the last repeater, place redstone on the top of it, and a row of blocks on the back of it until it will be horizontally inline with the water source. Place redstone on top of the row's ends (first and last blocks in the row), place an opaque block over the front redstone on the row and place repeaters facing towards the rearmost redstone dust on the row. These repeaters must also be on 4 tick delay. Place a block to the side of the latter (the block must be over the main trench) and place blocks until you'll be 2 blocks horizontally away from the mounting block. The last block must be a solid block. Then, place a piston facing towards the front of your cannon. It will push the shell. Where the shell will be, add 2 blocks on the sides. One of them must be a power-emitting block. Place redstone all over the new row of blocks, placing repeaters if needed (for delay).