Penetrating captive bolt followed by immediate exsanguination (bleeding out) is the preferred method for euthanasia of cattle in abattoirs (slaughter facilities). The mode of action of a penetrating captive bolt gun is concussion and trauma to the brain. This requires that it be held firmly against the surface of the head over the intended site. This constitutes a major difference between the placement of a firearm and the placement of a penetrating captive bolt. Because placement and positioning of the projectile is critical, some degree of restraint is required for proper use of this device.

There are two types of captive bolt: penetrating and non-penetrating. Both are discharged by gunpowder or compressed air. A penetrating captive bolt works by concussion and trauma to the brain. It causes immediate unconsciousness and destruction of brain tissue as a result of penetration of the discharged bolt. While the destruction of brain tissue with the penetrating captive bolt may be sufficient to result in death, operators are strongly advised to ensure death by exsanguination. The non-penetrating captive bolt device works by concussion

Penetrating captive bolt and charges and only stuns the animal. Since the destruction of brain tissue is minimal and level of consciousness more variable, it should not be used for euthanasia of livestock in field situations.

http://www.vetmed.ufl.edu/lacs/HumaneEuthanasia/gun.htm