Barbarus weighs in with the possibility that a round fired aft from a fast aircraft would simply be traveling slower relative to the planet and when that more limited velocity diminished, it would lose it's remaining velocity and fall.
I think this theory has merit. I am sure the Air Force has tested this and I am interested in finding the results if they have been declassified.
Aesop points out that the charts I found for 20mm were for air to ground, meaning that it was being fired in heavier air at low altitude, so the data on 20mm projectiles fired at 30,000 or 40,000 feet is still not known(to us).
In space everything's velocity is relative to the observer or between objects, and to some extent this is true between aircraft and projectiles. Thinking about a spacecraft traveling 1000 miles an hour and firing a projectile directly aft at 700 miles an hour. It stands to reason that the object would still traveling in the same direction as the spacecraft at 300 miles an hour. The difference if this was an aircraft would be the air that then begins to act on the object.
Any old AF gunnery experts among our readers?