Create player as pilot UH80 with AI gunners in the editor. Fly over large group of infantry. Especially happens in urban areas, where they won't fire at all. In open areas (I tested in the salt flats), they will engage, but will only fire in 5 round bursts or so, but will not hit hardly anything. This is on max AI skill.
Additionally, they will not fire if the aircraft is moving very much - you need to be almost in a hover for them to shoot. Forward motion more than 15-30km/h seems to prevent all firing.