Warning! A destructive device; use caution.
Step 1: Materials
Everything you see here is what you need to create this very easy Minecraft machine gun cannon. (Plus lots more tnt).
First make a stone structure like this and put 3 red stone torches on the underside. The opposite side of the torches is the way it will shoot.
Next put 6 dispensers down in a formation like this. Add 5 slabs below to block the water from flowing. Then place the water under the top dispensers. After this load up all of the dispensers to the brim with tnt.
Watch this final video to see how to activate the cannon and get shooting!