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I'm unsure if the Razorwing/Voidraven can fire 4 missiles in one turn or are limited to one missile per turn. Can anyone shed some light on this issue.
Here's all the relevant info I can find
1) Both vehicles have Aerial Assault so that they may both fire all of their weapons while moving at cruising speed.
2) PG 58 Rulebook. Optional weapons- some vehicles have, among the options of their army list entry, the possibility of buying additional weapons, such as one-shot missiles and pintle-mounted guns. Firing one of these additional weapons counts as firing one of the vehicle normal weapons.
So can you fire all or can you only fire one per turn?
The missiles are 4 individual weapons. As long as you are adhering to the vehicle-type and move distance (this one says you can fire all at cruising speed) then you are fine. A common thought on a tactic is to bring hte fighter on from reserves and dump all the missiles on a choice unit.
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As said above, as long as you're moving at cruising speed or less, fire away!
Note that if you do that, though, they all have to be fired at the same unit. Of course, when firing blast weapons you can put the blast over any model in the unit, so if your opponent has clustered a couple of units close to each other you could probably get some hits on other units (plus whatever scatter does for you).
They will fire as a Multiple Blasts, as 'Mijal' says. See page 30 of the Main 40k Rule Book.