They can shoot all the stands that can see the enemy (LOS).
so if it is a giant or (or short single base) only one can shoot. If it is a three base unit, some part of the unit will pop in the line of sight of the pistoliers son they can shoot the three bases.
Edit, ooooh yes, re reading the Dave answer, They can shoot in any moment during the charge. Dave answer is perfect.
But this is interesting for hellblaster because they can wait unit 1cm to contact and then fire with all the potential of the artillery
Is my opinion