My friend told me this not too long ago and I thought I'd share my insight with you guys.
Have you ever wondered why reserving the passenger seat is referred to as calling 'shotgun'? I never knew until recently. Apparently when cops go patrolling in their cars, their partners always have a shotgun in the passenger window. That's where the name was derived from.
i think it was from even Longer than that, i play Red Dead Redemption on the ps3 (gta style Cowboy game) and you can ride Shotgun in the Passanger Cart... i think its just like having protection and a shotgun works great. lol thats just my opinion,
EDIT: also now adays most cop cars have a shotgun on the door oposite the Driver. i have no idea why tho