Regarding the last post, I have to say that 30 x 30mm rounds should down a B17. Official testing data on the MK 108 gives some info on this. The engineers presumed that 4 hits with the MK 108 would down a B17 (1 hit should down a fighter). Double this numbers and you will be around 10 rounds for a B17.
each hit in wing caused a 1,75m hole (in radius). Any questions? If you need 30 rounds, the B17 is way too resistant.
Werewo
JG 301 "Heimatverteidigung"
P.S. Hopefully there will be a time when damage and gunnery model will represent the real potential of the cannons.