When in a tank or other vehicles, it is possible to shoot in a completely different direction to that of the direction you are facing, like when you let another player control the gun for you, they can shoot backwards while you drive forwards.
You don't need another player to do this for you, it's VERY simple for you to do it by yourself!

In Options, under Controls, there is an option to turn on CAMERA WILL FOLLOW VEHICLE TURRET!!

TURN THIS ON!!!

Now, when you move your tank turret to the left, the camera will not be stuck in position looking to the front of your tank, but will move according to the barrel! You'll be looking straight down your barrel at ALL times!!
Personally I think it is ABSOLUTELY essential to have this option turned ON!
When retreating back to base for repairs, you can still shoot at any enemies attacking you. You may even be able to kill them before they kill you.

OPTIONS -> CONTROLS -> LOOK/MOUSE -> CHECK BOX FOR 'LOCK VEHICLE CAMERA TO TURRET'

HAPPY TANK FRAGGIN!