In SDK's gun mod in Minecraft, what are cannons used for and how are they used?

Cannons are mainly used in defense especially in PvP mode they consist of obisidian or diamond frames (Obisidian frames are cheaper to make since you can farm obisidian)some redstone to activate the TNT 2 buttons and TNT of course learn more here:http://www.minecraftwiki.net/wiki/Cannons#Cannons