Well, I'm no expert in ballistics/energy in regards to WW2 weapons, but I do resemble one for rifle calibers.
Let's take a garden variety .300 win mag round. At 1000 meters, it still has well over 870 foot pounds of energy. That is a hell of a lot more than even a .44 magnum has at point blank range.
Now, take the .50 cal 750 grain Amax round that I shoot. It has nearly triple the energy of a .300 win mag at 1000 meters. Make no mistake, kiling APC's at this range is usually a one or two shot deal so long as shot placement is correct. And that is what this argument bakes down to IMO - shot placement.
In air to air combat, even while in level flight, there are SO many more variables when it comes to hitting the targets, I'm sure this isn't anything everyone here doesn't already know and accept.
However unlikely hitting another fighter at 1000 meters may or may not be, be certain that if a .50, much less a 20 mm explosive round hits said fighter, it will still have more than enough energy to do catostrophic damage, so long as it strikes the right place (fluke).
So, the original posts statement that killing a fighter in AH at long range is unrealistic because of the distance/power issue isn't very accurate, that said, I've never tried shooting from my 172, but I imagine it would be like anything else: difficult at first, much easier with experience when it comes to actually striking long range targets.
We have a Hispano 20mm in our shop, it came out of a Hurricane 2c at the local air museum. I should round up some 20mm ammo and take it to the range and shoot some tracer at an old car at 1200 meters or so. Video tape of that would prove the case one way or the other, at least in regards to inflictable damage at that range.
