This is a crude model of a fully functional TNT launcher that I created, you are free to extend it to as many cannons, so far 4 come pre-installed but please if you want to modify it and give it to others, then please give me credit :)

You can re-fill the TNT by putting them in the correct order as they are placed already when you get the map, this map is in Creative, if you can convert it to Survival that would be GREAT.

IF YOU DO NOT REFILL THE TNT'S PROPERLY THIS SYSTEM WILL MALFUNCTION AND BLOW EVERYTHING AWAY.

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UPDATE: NEW 8 CANNON LAUNCHER NOW AVAILABLE: www.planetminecraft.com/project/cannon-launcher-626-8-cannon-edition/
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Materials used:

Obsidian (a lot of it)
TNT
Cobblestone
Stone Slab
Redstone
Redstone Torches
Torches
Water
Wood
Signs
Glass Blocks
Glass Panes
Iron Door
Ladders

Those are the materials I used, at least those are the ones I remember using.

HERE IS THE BETA (MORE CANNONS VERSION): www.planetminecraft.com/project/cannon-launcher-626-more-cannons-edition/
-Ikutoisahobo

Additional Notes

NEW: www.planetminecraft.com/project/cannon-launcher-626-8-cannon-edition/