This tutorial will be showing you how to make both a simple TNT cannon and a full auto TNT cannon with dispensers.
Step 1: General Layout
These two pictures show the general layout of the TNT cannon with a bird's eye view onto it. If you are accustomed with Minecraft and understand all of this you can simply go ahead to creating it from here on, based on these pictures. For materials you will need the following:
-33 solid blocks, best are obsidian, end stone or bedrock if you are in creative mode.
-12 redstone dust
-8 TNT (for each individual explosion)
Make sure you have all these materials before beginning.
Step 2: Foundation
To start off you will build a 10x4 rectangle from some kind of solid blocks, best for this is obsidian as it will not break if the explosions sets of without the water stopping it from damage. Then you remove the strip of 8 blocks that you can see in the birds eye view. This is where later on the TNT is placed. Do not put in the TNT yet though. Continue by placing 2 more solid blocks onto the front two blocks off the left side. Next add a fence at the end off the gap and put a carpet/pressure plate on top off it. Add redstone in the places show . Next, add water to the very last block of the strip of air. If done correctly, you will end up with a water stream that goes right to the fence and stop there. Later on this will make sure the TNT does not blow up the blocks you use. If you want you can also add blocks underneath the water stream and fence.