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The bolt itself is a heavy rod made of non-rusting alloys, such as stainless steel. It is held in position inside the barrel of the stunner by means of rubber washers. The bolt is usually not visible in a stunner in good condition. The bolt is actuated by a trigger pull and is propelled forward by compressed air or by the discharge of a blank round ignited by a firing pin. After striking a shallow but forceful blow on the forehead of the animal, spring tension causes the bolt to recoil back into the barrel.
The captive bolt pistols are of three types: penetrating, non-penetrating, and free bolt. The use of penetrating captive bolts has, largely, been discontinued in commercial situations in order to minimize the risk of transmission of disease.
In the penetrating type, the stunner uses a pointed bolt which is propelled by pressurized air or a blank cartridge. The bolt penetrates the skull of the animal, enters the cranium, and catastrophically damages the cerebrum and part of the cerebellum. Due to concussion, destruction of vital centres of brain and an increase in intracranial pressure, the animal loses consciousness. This method is currently the most effective type of stunning, since it physically destroys brain matter (increasing the probability of a successful stun), while also leaving the brain stem intact (thus ensuring the heart continues to beat during the bleed. This however has no bearing on the efficacy or speed of the bleed, as cardiac output requires a venous return, as venous supply is severed, this does not occur. The bleed hence relies on gravity and not a heartbeat). One disadvantage of this method is that brain matter is allowed to enter the blood stream, possibly contaminating other tissue with bovine spongiform encephalopathy (BSE, colloquially known as mad cow disease).
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. This concussion is responsible for the unconsciousness of the animal. This type of stunner is less reliable at causing immediate and permanent unconsciousness than penetrating types; however, it has undergone a resurgence of popularity due to 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.
The free bolt stunner is used for the emergency, in-the-field euthanasia of large farm-animals that cannot be restrained. It differs from a true captive bolt gun in that the projectile is not retractable; it is similar in operation to a powder-actuated nail gun or conventional firearm. Capable of firing only when pressed firmly against a surface (typically the animal's forehead), the device fires a small projectile through the animal's skull. The veterinarian can then either leave the animal to die from the projectile wound, or administer lethal drugs.
With cattle, goats, sheep, and horses, a penetrating stunner is typically used since it destroys the cerebrum while leaving the brain stem intact; this results in a more consistently reliable stun, and ensures the animal's heart continues to beat during the bleeding process. Captive bolts allow for meat trimmings from the head to be salvaged. In some veal operations, a non-penetrating concussive stunner is used in order to preserve the brains for further processing.
Captive bolt stunners are safer to use in most red meat slaughter situations. There is no danger of ricochet or overpenetration as there is with regular firearms.
The cartridges typically use 2 to 3 grains (130 to 190 mg) of smokeless powder, but can use up to 7 grains (450 mg) in the case of large animals such as bulls. The velocity of the bolt is usually 55 metres per second (180 ft/s) in the case of small animals and 75 metres per second (250 ft/s) in the case of large animals.
There are certain specific stunning sites for various animals:
There seem to be at least two cases in which a bolt pistol is used as a murder weapon. One is recounted in a paper published in The American Journal of Forensic Medicine and Pathology. The murderer was a 46-year-old man who killed his wife and also had a history of alcohol abuse and aggressive behavior. The other case is the murder of Mary Griffiths. She had requested police help to deal with harassment from an acquaintance-turned-stalker. The police delayed dealing with the problem and the stalker killed Mary Griffiths, with the bolt pistol, in front of her child.
A bolt gun as murder weapon has also been featured in fictional works. In the novel No Country for Old Men a bolt pistol was the primary murder weapon of the main antagonist. In the US/Canadian television series Haven a multi-episode arc of the story concerned the pursuit of a serial killer known as the Bolt Gun Killer whose victims were dispatched by a bolt gun fired into the back of their skulls.
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