Since todays (12.10.2015) release of the development branch (1.53.132856) you are able to stand up and shoot in the driving/facing direction of the offroad.

To activate and deactivate that feature, use your mouse wheel or press the "turn out" and "turn in" keys (left ctrl-q and left ctrl-e by default).

If you stand up and facing to the front, you are able to lower your weapon too far. If you shoot, you won't hit the Offroad (no hit box).

Screenshots: http://imgur.com/a/21CB7