In the Arma saga for move turret and aim in the vehicles only need use mouse, but in the reality gun move is independent of the movement of the turret. For move turret you use your feets, or pedals, or one joystick in the right side (see the picture), and you can move the gun (not turret) around 40º. Implement this feature is easy, A and D keys move the turret inside the vehicle, and mouse move the gun.
http://www.controls.com/turret-control-systems/

http://www.hmmwvinscale.com/OGPK%20interior.jpg
http://www.warren.af.mil/shared/media/photodb/web/2013/01/130110-F-JW079-051.jpg

Videos
https://www.youtube.com/watch?v=LJJZMPvmxvM - Second 00:056