First off, have the guns watch in some direction with the dowatch command. "this dowatch getpos barrel" or something like that.
Next, in your script, use the line: "gun fire gunweapon", where gun is the name of the gun to fire, and gunweapon is the name of the weapon the gun fires. That will get the gun to just shoot where it's barrel is pointing (where it is looking).
Now last, if you DON"T want the guns to actually shoot anything, you could put this in their init field:
this addeventhandler["fired", {deletevehicle (nearestobject[_this select 0, _this select 4]}]
That will delete the shell right after it is fired, so it will be like the guns are firing blanks.