When Rambo kills the gunner on the jeep-mounted machine gun, and takes over the weapon, the first thing he does is lower the barrel and blow away the
soldiers in the forward seat. This would not be possible. In order to prevent
this from happening accidentally in the heat of combat, the gimbal on which the gun is mounted would have a built-in mechanical stop, which would prevent the gunner from lowering the barrel far enough to kill his own crew. This safeguard was applied on WWII bombers, to keep the side and turret gunners from shooting into the wings and engines of their own planes.