the action of a non-penetrating stunner is similar, but the bolt is blunt with a mushroom-shaped tip. the bolt strikes the forehead with great force and immediately retracts. the subsequent concussion is responsible for the unconsciousness of the animal. this type of stunner is less reliable at causing immediate unconsciousness than penetrating types; however, it has undergone a resurgence of popularity because of concerns about mad cow disease. in the european union, this captive bolt design is required for slaughter of animals that will be used for pharmaceutical manufacture.