Minecraft wiki entry for TNT_cannons, associated text: Remember that all repeaters must be set to their longest setting.
Note that obsidian was used here to reduce the possible damage in case of misfires, but feel free to use any other strong, solid block.
An improved fully automatic shotgun style cannon tutorial and demo can be found here [6]
Make sure all the dispensers face into the barrel of the gun and remember that only the first water block at the back is a source block.
Also by using pipes of hoppers attached to the backs of the dispensers, one can more easily distribute stacks of TNT from a central point, such as a chest.
Feel free as always to improve the design. Happy hunting!