The commander orders the driver to point the whole vehicle into a certain directon. Then the gunner aims at the target with the swivel range the gun has. If he doesn´t get the target into the swivel range, he reports to the commander/ directly to the driver if needed that the whole assault gun should be slightly turned left or right, so he can engage the target.
I think that´s one of the reasons asault-guns were better suited to set ambushes than actually assaulting. It was way easier to set ambushes with them, also because of the low silhouette.