A .50bmg is going to punch .5" holes in metal structures that it can penetrate. If one of those passes through the aluminum skin of the wing without striking a fuel tank, or a gun etc. it is really nothing more then a pinsalamander to the airplane. In real life, in order to destroy airplanes, guns like the .50 cal relied on striking critical structures within the airplane - fuel tanks, oil lines, radiators and coolant lines, engines, pilots, control cables etc. Of course, airplane designers did their best to protect these things. They put armor on the back of the pilots seat, installed self sealing fuel tanks etc.
I have a copy of an RAF test of vulnerability of the Bf109. They shot .303, .50, and 20mm rounds at it from a variety of angles. They found that from dead 6, the .50 couldn't penetrate the pilots armor at 100 yards because it was passing through the fueselage structures and the gas tank before it got there. Since the fuel tanks were self sealing they could withstand a few individual hits without causing major leaks. The engine was almost impossible to damage from this angle, because any strikes to it were at such a low angle of incidence they would just ricochet off. The radiators were very vulnerable however, and any strikes to them resulted in leaks which would cause the engine to overheat in a matter of minutes. So in order to shoot one down from that angle, you had to either score a hit on the radiator, or enough fueselage strikes that either the gas tank started leaking and caught fire, or the pilot's armor failed from being battered by enough hits - this meant getting close and hitting them with a LOT of bullets.
Interestingly, the 20mm cannons weren't much different - the HE rounds were better against the fuel tank but still not a 1 hit guarantee of fire, and the AP rounds were better at penetrating the pilots armor. The causes of the airplane going down were the same - it just required fewer rounds to do it.
Of course from different angles the story changed - at higher deflection angles the pilot and engine were now more vulnerable, and a larger cross-section of the radiator was visible as well, now you had to make a much more difficult deflection shot to hit the target to begin with.
Now in AH we have more of a "hit point" oriented system where planes primarily get shot down by having big pieces sawed off (wings, tails etc). But the end result seems to be about the same - if you get in close and score a solid 1-2 second burst, it's usually a kill. The cannons generally kill more quickly.
