I just found out that in the new minecraft 1.7.2 you can make cannons with commandblock using the /summon command all you have to do is get a commandblock by typing /give (name) 137 then after that you right click it then after that you type in /summon PrimedTnt x:(Coordinates) y:(Coordinates) z:(Coordinates) then after that get a button/lever/redstone torch/pressureplate or tripwire hook and that will activate the commandblock then it will shoot a tnt in where the coordinates is and this is pretty useful for adventure maps with pirate ships and for griefing :3 you can always make a rapid explosion of this thing by doing the way to make a machine gun thing you can also type /summon PrimedTnt rapidly to fly :3 but it can damage you

~A Diamond Please? Peace Out :)