From the documentary
Tom Neil @ 39:10

Re: 20mm

"2-3 shots"

12-20 for downing a bomber perhaps.

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

Are we talking about how many are fired or how many hit?

I think it was the US that did a study in WWII that said that 4500 rounds had to be fired to bring down a plane. I am guessing they were talking about .50 cal since that was the standard (depends on when they did the study though). Obviously they weren't talking about how many hit the plane but about how many rounds were expanded in comparison to kills. Even then, that number sounds low.

In a single engine fighter, obviously one bullet could do the trick. In a bomber, with a co-pilot, there is obviously a much lower chance of one shot kills. That's why the Germans in particular kept experimenting with larger caliber cannons for bomber interception. (several countries did)

Bullets do strange things.

Splitter