I'm actually a little curious how screwy bullets fly when they're shot perpendicular to a high speed. Door gunners on a chopper are the closest I can think of, but I'm not sure how accurate they are in general and I don't know how less a 7.62 round would lose course vs a 9mm.
Edit: The guns on an AC-130 or any comparable craft would probably be moving faster, but from my vast military knowledge of Call of Duty, I know that the rounds they fire are even larger.
It absolutely does. Airplanes were being shot down in Vietnam with an SKS firing 7.62 rounds. Now, the question at that point would be do you want someone lobbing bullets into the sky so they can fall back down and potentially hurt someone or damage something? Absolutely not. But it's not only possible, it has been done before.
Did you know that Mythbusters tested whether or not an airplane would take off from a treadmill and came to the conclusion that a plane rlies on air speed and not ground speed?