Exclaimation used when one specifically does not want the shotgun seat in a car, most often in a cab. The last person to call "notgun" has to sit upfront with the cabbie and will most likely get stiffed with the fare.
"Ha ha, Lance forgot to call notgun and is now stuck upfront with the cabbie. Let's stiff him."
Exclamation used to denote any space in the backseat of a car. One typically calls "buckshot" only after having been beaten to shotgun.
Buckshot can be used specify a particular rear seat by adding "right" "left" or (rarely) "center" after buckshot. This is done in order to avoid sitting behing a fully reclined seat or getting stiffed with the middle.
Tall and Fat Bill: Alright guys, let's go to the beer store, I'm driving.
Jimmy: (so as to avoid sitting behing Tall and Fat bill)BUCKSHOT RIGHT!!