In the old Wild West, a .45 cartridge for a ‘six-gun’ (revolver) cost 12 cents—so that was the price of a ‘shot’. But a glass of whiskey at a Wild West saloon also cost 12 cents—so it was easy to conflate the two. If a patron was low on cash, he would instead offer the bartender a cartridge in exchange for a drink. This became known as a ‘shot’ of whiskey.
