based on..?
what is the meaning of forward facing?
Forward facing means just that- weapons that are facing the front end of a vehicle. Most fixed weapon systems are like that in the game.
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Weapons systems that are "fixed" (ie: Great Grey mg, Hurricane Walker's guns, Dragonbike systems, GT Offroad mg) only have a 90 degree arc of fire to the front (45 degrees to either side of the centerline of the front of model).
Weapons that are described to be in a turret are 360 degrees unless restricted in turret movement. (TA-6500, Vermin APC)
Walkers with weapons in arms (Meka, Eradicator) have a Limited fire-arc like infantry (ie: 180 degrees or) 90 degrees to either side of center-front.
In the case of bikes and open top vehicles like the GT, if the crew has guns and does not have the arc for the main weapon, they may fire their personal guns using one action as detailed in the rules (iirc).