Hello everyone :)
Some time ago I created a huge cannon made out of obsidian which was supposed to fire TNT. Back then it was only possible to accomplish this using dispensers and a lot of redstone wiring.

With the 1.7 however(And the introduction of the /summon command), this cannon was completely re-done! The only thing that is still similiar is the design and the fact that it shoots TNT.

Features:
-Fires TNT by simply pressing a button
-3 different power modes that will make the primed TNT travel different distances(weak,normal,strong)
-A special overdrive mode that will make the cannon shoot TNT continously
-Unlimited ammo, you will never have to refill the cannon!
-Fancy design

All you have to do to use it is download the schematic(s) and place it in your world(Do NOT rotate it*). Now you can climb onto it and try it out, it's very simple.

*Important sidenote:
The cannon that you get from the schematic from PMC needs to face east all the time otherwise it wouldn't work(Because of the way the /summon command works). If you want it to face any other direction you can download a folder with schematics for all the 4 directions. If you rotate the schematic it won't work so always make sure to use the proper one ;)

Here you can get all the schematics:
www.mediafire.com/download/jforgpau8cywa1w/4cannons.zip

I also prepared a world where you can test the cannon against some targets. It's nothing special but the world also includes the "Human Cannon" which shoots you straight up in the air which is, as I think, a nice feature.