Fracas,
Well, actually you can! We answered this a while back. It's the same for Ork Heavy Gunz. All gunnery weapons from a squadron get to fire at once, however you calculate thier dice on the gunnery table seperately for each type of gunnery weapon. Meaning they don't suffer from gunnery shifts due to BM's caused by other members of the same squadron.
I can't find a reference for this so I'll have to ask the rest of the HA.
Cheers,
RayB HA