Troops aboard vehicles can fire from firing slits, hatches or over the sides of the fighting compartment if the vehicle is open-topped.
All of the models aboard an open-topped vehicle can fire.
Fire Points. Each enclosed vehicle has a number of fire points defined. A fire point is a hatch or gun slit from which one or more passengers inside the vehicle can fire. The number of fire points on each vehicle and how many models can fire from them is specified in the 2003 Chapter Approved compilation.
Troops firing from a vehicle count as moving if the vehicle moves and may not fire if the vehicle moves more than 12". This means they may not fire heavy weapons from a moving vehicle unless normally allowed to fire heavy weapons while moving.
If passengers fire from a closed vehicle and do not have an Armour save of 3+ or better, then in the following turn enemy firing at the vehicle count it as open-topped.