ride shotgun

[kjc0123]

I know "to ride shotgun" means "to sit next to the driver in the front part of a car, or to say you want to do this" but why does the idiom come to have such meaning? 

[Clanad]

Stagecoach guards rode shotgun � they just didn't call it that in the 1880s, as far as anyone has yet discovered. The term "riding shotgun" to refers to the guard sitting next to the driver doesn't emerge from the Old West but rather from movies and TV shows about the Old West. To date no one has found a cite for "riding shotgun" during the time stagecoaches were actually used.
A shotgun scattered pellets, making it easy to hit your target at short range. If the barrels were sawed off, the shot scattered over a wider area, an advantage if you were defending against a group (robbers or wolves, for instance). You didn't need to aim precisely, just point the gun in the right direction.

The Stevens Point Journal (Wisconsin) from September 9, 1891, says: "Of all the devices and inventions for the protection of treasure and circumvention of the road agent, the only one that has stood the test of time and experience is a big, ugly-tempered man with a sawed-off shotgun on the box � It is the business of the man with the sawed-off shotgun not to let the robber get the drop, but to blaze away as soon as he shows up. The gun is sawed off for the greater convenience of the messenger in taking care of road agents. It is loaded with buckshot and scatters like a charge of bribery fired into a California legislature." (Love their way with words back then)...
In the classic 1939 movie Stagecoach: Curly, the sheriff, says, "I'm gonna ride shotgun," and John Wayne expresses surprise at seeing him in fact riding shotgun later. So we have references from pulp fiction and from the movies (but not from the Old West itself) using the term "riding shotgun" to refer to the stagecoach guard...

[david.n.c]

Shotgun...Double barreles. thats what i was told, you were esentially the second barrel

[cadama]

ride shotgun comes from places where it's not safe to travel unarmed. Of course the driver can't drive and defend himself so he has a passenger in the front seat with a weapon to return fire if they are fired upon.