That's an easy one:
The primary fire button fires only 2 guns, the secondary fire button the other 2. So if you press primary fire only, you will shoot until those 2 guns run out of ammunition. That's why rearming "fixes" it.
To shoot all guns at once, use the "fire all" button.
(The "real" Wirbelwind worked the same way: The gunner had two pedals, each firing a set of two guns)