Actually, I thought that the guns whine was a good place to point out a "conspiracy". Why do the German guns suck so bad? Well, because of the way HTC chose to model the rounds. If I understand the modelling right, we use a "hybrid" round with the "stats" averaged out between the rounds and I suppose they have information on the ammo belting to use for a ratio. The only ammo belting I know was for the German planes, so I am not informed enough to give more than a rough example. But, for my rough example, lets take the Hispano 20mm. It was a large round with high muzzle velocity (130 grams at 860 M/sec for the Mk II). According to Tony Williams (who is a ballistics expert, as far as I know), the combination of kinetic energy and chemical energy rates a "damage" rating of 201. As far as I know, this damage rating is relative to the other guns, but you'd have to ask him to be sure.
Now lets take the German 20mm Mg151. I believe the Germans used a 2 AP, 2 HE, 1 Mine belt on the Western Front, and I've heard the proportion of Mine rounds was greater on the Eastern Front (although I don't know for certain). The MG151 AP round was 117 grams with a MV of 720 M/sec. So right away it is perfectly obvious that this round isn't going to do as much damage as a Hispano 20mm (its smaller and it is going slower). And sure enough, the "damage rating" on that round is only 110. The HE round has a damage rating of 109. The Mine round is where it gets good, in my opinion. It was 92 grams (of which 22% was explosive content), moving at 800 M/sec. Mr. Williams rates this round as a 236 for damage.
Now, to take this discussion out of the real world and back into the game, I've been flying German planes for a while now. I'd say I spend at least 75% of my time in them. Over the course of two years, I have NEVER seen 1 Mg151 round kill a plane. Over the course of one sortie in a Spit, I'll get at least one kill by taking someone's tail off with 1 round (either the vertical stabilizer or both horizontal stabilizers). With somewhat less frequency, I've gotten one hit kills in other cannon equipped planes. Now, one might say "Well of course you have never killed someone in one hit with the Mg151, it only hits about half as hard a the Hispano did". And you are absolutely correct. However, if the rounds were modelled as HE, AP, Mine, you would at least have a shot at landing one of the really good Mine rounds every time you fired. As it is now, the German guns are effectively nuetered by the way the ammo is modelled. I don't have enough information on Soviet or Japanese ammunition to say if it affects them as well, but I will say that the La-7s and N1Ks cannons hit harder than the Mg151 does, in my opinion.