Take any sniper rifle into the Hidden Valley level. Creep to the area where the truck stops and watch the sniper in the tower. As he is stretching, he holds the custom rifle out of the window. Shoot him in the side and he will drop the rifle out of the window. If it drops on the roof, just restart and try shooting him further up in his side, The M195 rifle is recommended, as does shooting him in the armpit.
