Algodoo

Yes, I know it's ugly
w/s to aim, a to activate. Turret will start firing as soon as any object moves near it.
It uses velocity meter coupled with long,attracting arm. The arm's attraction stabilises it in some position, and then any change of surrounding will change that position, causing the velocity meter to move tiny bit.
The turret is generally non-aiming, it will aim only when the target is only massive object nearby.