"of their weapons" they can fire up to the number of weapons that the unit/model has "using their full ballistic skill". The rule is about Ballistic Skill, it is not about how many weapons can be fired. The number of weapons it can fire remains the same as any other vehicle.
@Power Klawz, I'm not adding anything extra to the rules, I'm simply pointing out that the above rule has been misinterpreted. Those disagreeing with me are the ones who are dragging in everything(plus the kitchen sink) to avoid re-reading that rule(and possibly realizing they were wrong)...