The cannon is from the Balkon's Weapon Mod. It's fairly cheap to make, and works much like a ghast's fireball would.
===To load the cannon, simply have gunpowder in your inventory, and right click on the cannon with a cannonball. Wait a few seconds, and then hit the spacebar to fire. The cannonball and the gunpowder will be removed from your inventory each time you shoot.
this is useful if you're killing several enemies
If you load the cannon and wait too long, the cannonball will not fire and will blow up the cannon.