I've done this many times, place an empty, west jeep with machine gun outside the camera site, with a name, lets say T1, and either put this in the units init line
This dofire T1
Or name the guys, and put it in a trigger's on activation
Soldier dofire T1
However, this will make him keep firing until he's used up all his ammo, and generaly any grenades he has.
So if you only want him to fire one clip, I suggest removing all his weapons and ammo wit
Removeallweapons this (I think thats the command)
Add one magazine first, and then the weapon.
And he will do what you want.
I hope that helps do what you want.
EDIT
I almost forgot, the jeep must be at a 90 degree angle to the guy shooting at it, this will basically make him fire at the jeep, but not actually hit anything, thus making no noise of hitting the jeep.