I never really gave much credit on how many rounds it would take to shoot down a plane, there are so many factors involved that if you consider it under a simple physics point of view it's impossible to ascertain how many rounds it would take. Just to give you an example, there are at least two documented cases of Macchi 200 shooting down a B-17, and as you know they only had two 12.7mm machine guns that were actually crap, so go figure!

SJ