People, people. For the love of god, stop complaining about teh manouverability. Out all new flight physics will be included in 0.6 and will be historically correct.
You can down anything with ease, even with a .30 or .50 if you know where you have to shoot. The betty/HE-111 and most other bombers had huge fuel reservoirs in the back of their wings. Aim there for an easy kill. With fighters & light bombers you should probably aim for the engine.
I also suggest you study plane blueprints to see where the fuel tanks are. If hit there and the fual tanks aren't self sealing a single bullet will make the plane go up in flames! I'm not kidding you. Search the vulnerable spot for yourself, fire with your rifle or handweapon on it and see it go boooooooom.
And damage is also dependant on the frame of the plane. Most Russian planes will go down after 2 20mm hits because of their fragile wooden frame. American & German planes with a frame in aluminium will be able to take more punishment before going down. And the IL-2 will even be able to shrug off 20mm shells like in real life ;)