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).

To reproduce following issue, I recommend to use the keys.

If you now "turn in" and immediately choose or press "turn out", your character sits down and is facing to the front. You can now shoot through the Offroad (no collision).

Screenshots: http://imgur.com/a/J7sUo