I play SHOGO for the first time now, and noticed that it's never mentioned in the game's manual how to zoom in with weapons.
So if you'd like to know how it works, here goes:
Up to now, I found two weapons which can zoom in. The sniper rifle (obviously) and the assault rifle (which you have when not in your mech). If you want to zoom in with these guns, just press the button to select them twice (it's '6' for the sniper and '5' for the assault rifle). First button push selects the weapon, second push activates the zoom (third push deactivates it again).
Have fun with your headshots ;-)

Thank a lot for this.

Can it be bound to a standard key?

I'm sure it can be, but honestly the zoom is kinda useless. I never knew anyone to use it back in the day when everyone was playing. It doesn't work like it does in modern day shooters where the sniper deals tons of damage and all that...

This.

Thanks for the info!

I was actually googing this question!