Minecraft wiki entry for TNT_cannons, associated text: Make a long tube of obsidian (works best with 3x3x6 or 3x3x7) with a hollow middle. Then, put two blocks of obsidian on the top. Hollow it out. Fill with TNT, and put an non-blast resistant block in the last space on the front. Cover the top with redstone, and place a button on the back. Optionally, make a platform to stand on when you trigger it. You should also make it a little ways off the ground. See diagram below, in which the orange stands for the wadding (shown in cutaway side view):