After revisiting some of my old posts, I decided to simplify and extend the auto-turret.
You can select any one of your multiple targets, and you can have a cannon/tank etc made from a single rigged skinmesh.

A fun new toy. A mouse aimed tank.

This little tank has been rigged so that it has bones at the most important places.
The turret and the cannon rotate around their own bones, which is controlled by script.
The whole idea behind this exercise is to use a single mesh that has multiple movable parts, in order to conserve resources for projects which require a lot of game objects.

Left Mouse Button: Click on a ball to make it the active target - The tank will then automatically keep targeting the ball
Right Mouse button: Fire the cannon, or Aim and Fire if a target has not been set with Left Mouse
Drive as a normal car with Up,Down, Left and Right arrow keys

Enjoy