Fastzoom (observe, not nozoom)
To be able to use fastzoom you must know where your crosshair is without zooming, which you only can achieve by practicing (I'd suggest aiming on a target without zoom, then checking how right you were).
Fastzoom is used this way; you zoom, and fire shortly after. You can of course fire almost immediately or you can wait with firing, well, you see the point.

Download the fastzoom video

Trick for hitting nozoom shots
Everyone has experienced that pure nozoom shots seldom hit their targets, but there is a trick for this. Let's say that three or more opponents are rushing towards you or such & you fire a shot on the first one with zoom, and then you keep holding the trigger