Pistol whipping, or buffaloing, is not necessarily a weapon, as much as it is a move or attack that can be done using a typical handgun. It uses the handgun as a blunt weapon by holding the weapon normally, then smashing the barrel on the opponents head. However, other methods include holding the barrel of the pistol and smashing with the butt of the weapon, but this method can harm both the opponent and the attacker if the gun still has bullets in it.
The first guns like the Hand Cannon were heavy enough to be used as clubs, some had sharp handles that would double as a spike. Pistols like the Pirate's flintlocks would be used as clubs after they run out of ammo, which was common for single shot. On the battlefield, the idea of using rifles or muskets as blunt weapons was frequently replaced by bayonets. However since the beginning of the use of guns to even today; it is possible to use a gun as a somewhat effective blunt weapon for both self defense and assault.
The Pistol Whip was the Special Weapon of Jesse James , and was in fact re-popularized by him in the mid 1800's.
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According to the show, a well aimed and powerful blow to the skull can instantly kill a person. This doesn't mean all head blows will be instantly lethal, as the simulated fight showed.