If you're out of ammo for any weapon other than the laser and you still have ammo for laser or another gun you can fire the weapon by pressing the left mouse button while keeping the right button pressed.
For example if you're out of ammo for the flame-thrower, hold down the right button, press the left one and you will fire your flamethrower while your grenades-ammo will decrease with 1.
If you only have laser you can fire the flamethrower and it will only cost you one laser-shot. This works EVERY time for any weapon other than laser!
