WELCOME ABOARD!
All of the shells for the flak guns are good, as far as I know. As for making them more powerful, I'm sure it can be done but I'm not sure how. Somebody will be along who knows. I go with the historical answer, though. For me the best anti-aircraft weapon is deep water; I dive and stay there until the evil sub-killing monster is gone.
About the security questions: Every time we relax it even a little bit we get swamped - and I mean overwhelmed - with spambots, as well as human spammers. The site owner has done his best to limit those, and the price is that you really have to want to join these forums to get in.