Sorry, me again:
3rd person crosshair -> has any one done it?

What I mean is: the camera sits behind the player somewhere, and a little above. if you shoot, the mouse cannot give a unique 3d position, instead it gives you an infinite line of points that the player could shoot at.

If a target is selected then it's no problem: the distance between camera and the targeted enemy is the depth of the vec_for_screen function. But what if no enemy is targeted?

c_trace won't do, as I'm talking about a space shooter. there'll be nothing that can be hit, in most directions. And since the weapons are not all instant-hit-weapons, the player needs to aim somewhere infront of the moving enemy ship, not directly at the ship, in order to hit him.

Any suggestions?