YES I TOLD YOU ALREADY, Just make it just like the horse where when you mount it, you control it like a normal player and restrict movement.
Unless you make two shapes for the base of the turret and the turret itself, you will need to restrict vertical axis rotation of your view because it would look wierd. If you do have two shapes, base and turret, you can place the base as the turret spawn or something and just make the turret shape a vehicle with all control except movement.