Minecraft wiki entry for TNT_cannons, associated text: Pros: The time taken to reload TNT is at its absolute minimum. All a player has to do is to make sure the next shot is placed after the first shot is fired to make sure you don't destroy your cannon. This cannon requires little skill at all to fire; the hardest part will be placing the cannon a proper distance away to make sure you don't over/under-shoot. Such precision makes this cannon good for castle defense. Furthermore, through use of comparators, it is possible to make an easily aim-able dispenser-based cannon, and dispenser-based cannons do not blow themselves up if hit with another TNT cannon.