Okay, accuracy of the aircraft weapons is paramount. To that end this involves modeling every round, ensuring accurate trajectories, using accurate ammo sequencing (belting), and providing an accurate collision detection and terminal ballistics system... ...
I think we have to ask ourselves how real do we want the game to be. Most people who play this game seriously would say as real as possible. I think the BF109 is extreme realistic!
Most people who have prior iL2 experience would also say that in certain situations not the the game is the problem, but the pilot. Depending the accuracy of deflection shooting, about 30 MG17 S.m.K.h.armor piercing projectiles per second are more than enough for a PK, even in Cliffs of Dover. Under certain condition, S.m.K.h./AP rounds punch clear through the aircraft, hopefully through something critical, at best a pilot kill.
Information concerning the real life ammunition of Luftwaffe you find here:
Regards Varrattu
(Do please excuse the grammatical errors, English is not my mother tongue.)