"Most slaughter establishments use penetrative captive bolt stun guns to render cattle unconscious, quickly and painlessly prior to slaughter. Penetrative captive bolt stun guns have steel bolts, powered by either compressed air or a blank cartridge. The bolt is driven into the animal's brain. In the past, captive bolt stun guns were often built or modified to inject compressed air into the cranium of cattle, so as to disrupt the brain structures and induce total and prolonged unconsciousness, to ensure that cattle were slaughtered in a humane manner. Studies have shown that penetrative captive bolt stunners that incorporate air-injection can force visible pieces of brain and other CNS tissue into the circulatory system of stunned cattle.