from the manual (http://www.planetunreal.com/infiltration/manual/).
Weapons Mode Key:
Many of the weapons featured in Infiltration are designed to push usability and functionality to the limit. Some weapons feature full automatic fire, but can switch to burst rated fire, or even semi-automatic mode. This key is assigned to perform these kinds of functions. It also serves as the zoom adjustment key for sniper weapons. Investigate your weapons closely, as many of them will have direct uses for this key.
Assign it like you do any other key/function.
<IMG SRC="http://www.lumiere.net/~cloud/sig.jpg"">
I make dead people.