In version 1.18 there is a condition command called:
mousestate=x
I am not sure how this works. However, I am guessing x will be 1 for left mouse button, 2 will be for right mouse button.
The problem will be that pressing the left button will always fire the gun for those of you who don't want the gun to fire.
So, I would guess this would be the script
:state=x,mousestate=1:actions here
Because this is a condition you could have,
:mousestate=1:actions
however, that might cause a real problem with fps or you may not want the actions to be fired everytime the player presses the left button. It should be associate with at least a state if not another condition.
Another issue is that I'm not sure if this will work with FPSC. It might work only with FPGC (without guns). All you can do is try it.
BTW FYI, this should have been in the "FPSC Scripts" forum.

Terry