I don;t know why it's set up this way, but when you jump into a gunner position the fire primary work work the gun. if you want to fire all the guns, you have to have a new trigger set up that = Fire all guns. I don't have (or want) three trigger set ups.
My wish is that when you jump to a gunner position the default = primary trigger works the gun, and the secondary weapon trigger fires all guns if there are multiple guns (buffs, and 410) with having to program special trigger set ups for that.