This cannon has 14 dispensers. it is capable of sustained fire, and is designed to stop mobs, wolves, and players. configure it to your needs. the cannons on top blast down trees and walls, and those on bottom sear the ground.
If you need additional help, just ask. remember, it takes 3584 fire charges to fully load it. redstone HAS to be broken to turn it off. wait for the clock to empty, then replace it. soon i will build a new version. it will include an auto stopper and i will create a decorative dragon head.

Additional Notes

thanks to Chickenkeeper for teaching me how to make an oscillating circuit.