My understanding of ballistics is limited but I do have something to say that is pertinent i think: You are so right and wrong at the same time about that!

Point 1:

12.7mm APs rounds goes in and out against tin skinned part of a plane, just like a 20mm AP so thats not really a issue here. Again, you are so right, but it is when they hit something in-between that counts, all other rounds are wasted, just like you state, but the .50 has so many more chances to get that one critical hit!

Point 2: For the engine and armored part, that doesn't give any advantage to the 12,7 AP cause the 20mm AP will do more damage anyway. Again, you are so right, but the .50 has so many more chances to get that one critical hit!

Point 3:
20mm Explosive rounds do a 20mm hole on one side and multiples smaller holes on the other and release several cubics meters of hot flaming gas in the middle of that.  I am not a expert but I am pretty sure that overpressure alone could do massive damage on its own agaist most part of fighter and bomber. (think about firing a gun in a car with closed window...) I'm so glad that you admitted that you don't know anything about this topic! First the amount of hot gas generated by a 2.6 to 6.5 gram charge of any explosive is measured in liters, dozens, not hundreds! The shell often times explodes just under the skin and the only visible holes are with in 18" of the point of detonation! At that distance there will be many fragments that failed to perforate the thin skin of the target AC! Look at any photos to see this for your self. You can see the dents left by failure to hole fragments. Secondly the 20 MM Hisso shell only makes ten to twelve large fragments that have the power to perforate the skin and rarely anything thicker! Lastly, I have fired many guns from inside cars, some (MANY!) that generate more gas than the smallest Hisso shell, it is not nearly as bad as

thanks Aussiegunneragain. I think your picture confirm the effect of overpressure i was talking about isn't it?

 45-shooter, before jumping on a other subject you should acknowledge that your point of 12.7 mm superiority over the 20 mm had been proven wrong... That would actually increase what left of your credibility here.

 

Apparently you haven't heard of the Colt M-39 in while conducting your "expert research" ... sorry, I'm smirking.

They did not switch until the advent of the M-61 Vulcan gun in 20 MM! Note that the .50 worked more than "Well enough" in Korea Vs a plane every one knows is tougher than any in WW-II!

http://www.daveswarbirds.com/b-17/photos/body/side2.gif" width="375" height="233" />

 A German 20mm shell peeled the metal covering off this bomber like skin off an onion. Note the small caliber bullet holes in the fuselage star

 

They did not switch until the advent of the M-61 Vulcan gun in 20 MM! Note that the .50 worked more than "Well enough" in Korea Vs a plane every one knows is tougher than any in WW-II!

 

Then read this, if you prefer. It tells us how 20mm and 30 cal fire blew half the flight deck off a B-17G. I figure it might have been the 20mm's that did the damage, don't you?

 

PS, the damage far from the 20mm strike on the previous