This tutorial will be showing you how to make both a simple TNT cannon and a full auto TNT cannon with dispensers.
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.