Guncam footage - why aren't those gunners doing anything?

[Azimech]

Here is some collected footage, the part that is interesting is a bf-110 G2 attacking a B-17 from the rear, starting at 1:45. The rear and ventral gunners don't seem to fire back and it's possible they were out of ammo or already killed. The B-17 received much more hits than possible in IL2 1946 without catching fire.

http://www.youtube.com/watch?v=8oA9zy-qtxA