I'll tell you a secret. No one who knows how to shoot in the osti (or the M16 for that matter) even uses a gunsight. It is less than useless, it actually lets the gunner think he can hit targets further out than the game actually will allow.
And, I'm not kidding about this. If you want to know how it's done, search on this topic in the forum for a thread by Zazen, the undisputed master of all ground-to-air gunnery in AH. He describes how to setup without the gunsight, why it works, and at what distances you can kill.
Or, you can look me up in-game sometime and I can show you how it's done. It works, and it will surprise you, but once explained, it will also make sense.