I read in "Bomber", by Len Deighton, that ON AVERAGE it took twenty 20mm hits to bring down a four-engine British bomber. A good or lucky burst could bring one down with far fewer. Hits scattered all over a plane might damage it severely, but not bring it down. We've all seen the pitcures of the B-17 that took a direct 88mm hit in the NOSE and made it back. How many others flew on after 88mm hits, and would have made it home, only to be finally brought down by fighters? Oberleutnant Stigler, mentioned above, says he needed 7-8 well-placed 30mm (note: 30mm) shells to take off a B-17s wing. Given the slow rate of fire of 30mm canons, that's not an easy thing to do. In Roald Dahl's account of flying a Hurricane against the Luftwaffe in the Mediterranian in 1941, "Going Solo", he recounts making a way-too-fast diving attack on a Ju-88 over Greece. He fired off a burst, sure that he'd missed, but when he looked back, the Ju-88 was in a vertical death-plunge. He guesses that he killed the pilot, who then slumped over the controls. So perhaps this buff was brought down by just one or a handful of .303 rounds. Another book I read years ago told of a P-47 pilot whose plane was riddled by a Bf-109. The German ran out of ammo, pulled alongside the Thunderbolt, saluted and flew off.
So what's the point of all this?

A realistic damage model will sometimes seem unfair because even though you "emptied the entire load from my 109 into him", you didn't hit the right places often enough. [Or if you did, that packet got dropped by the system. Also, you probably didn't hit with close to 100%. If you put 50% of your shells into a plane you'd be an outstanding shot.] Or it will seem unfair because one burst exploded your plane. Most of the time, neither result will occur.
As for Karnak's downing of a Lancaster with 12 50 cal. rounds: By lining up four parallel guns along a leading edge to trailing edge segment of a wing from 15 yds away, he's given the P-38 its best possible chance for taking off the wing. The AH damage model, it is my understanding, will allow those bullets to penetrate into the wing, causing considerable damage. Still, 12 rounds seems like too few to cause such major structural damage. Certainly, being in such a superb position in actual combat would be extremely rare. However, if I can find some volunteers to fly buffs for me to shoot down without evading or firing back, perhaps more tests like Karnak's would expose some unrealistic aspects of the aircraft damage model.
/S/ MRPLUTO, Captain, VMF-323 "Death Rattlers"