Bukkit

Simple Cannon

To create your cannon, place a row of blocks in the shooting direction. This will be your cannon. The length of a cannon can be limited in the config. It's standard value is 1 to 3. Note that only certain blocks can be part of a cannon. They can be configured as well. Standard blocks are cobblestone, iron blocks and obsidian. Now place a wall sign at the end of the row on the cannons last block, and write "c" or "[cannon]" in the top line. Thats your cannon. Right-click the sign to shoot.

Rotatable Cannon

To make your cannon rotatable, you have to place a rotation block on the first cannon block (the one with sign). The default type is redstonetorch. Right- / leftclicking this block will make the cannon turn 90° right / left. If there is no space, nothing happens.

Redstone Trigger

To trigger your cannon with redstone, you have to place redstone (or repeaters/redstone torches) under the first block of your cannon (as shown in the picture).
Whenever the redstone (or repeater/redstone torch) is changing its state (on/off), it triggers the cannon.
If the cooldown is set to cannon depending cooldown, the cannon will not shoot while cooling down.

This feature is set off by default, it can be used to easily bypass permissions, ammo and player depending cooldown. To enable it, set "useredstonetriggers:" in the config to "true".
Please note that just placing a torch will not trigger a cannon, the redstone object needs to change its state!