[Strategerist]

In World War II I believe the figure was 8,000 German 88 Anti-aircraft rounds fired(mind you, these are big shells that airburst so you don't need a direct hit) for each Allied Bomber shot down.

The threat from terrorists shooting 50 calibers at airborne aircraft is for all intents and purposes, non-existent.

[GSlob]

Well, if the perp positions oneself directly under the line of approach to the plane landing at airport, his aiming would admittedly become easier. Still, even in such a case the perp (in single shot rifle mode) would need real time target distance and velocity data with instant recalculation regarding his projectile's time of flight. Since the perp would not enjoy the luxury of repeated practice to work it out, his chances of hitting his target on the first attempt would be extremely low, unless he gets a machine gun with a lot of tracer ammo.