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 replaced by bayonets.
The Pistol Whip was the Special Weapon of Jesse James , and was in fact re-popularized by him in the mid 1800's.
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.