fucking...
STOP RAPING ENGLISH!
The term SHOTGUN means that you wish to accompany the person doing said acticity. Saying "shotty not" means you're going to stand next to the person who's not doing it... it doesn't mean you're actually out of doing it yourself. The term SHOTTY NOT makes no sense at all, and it should be illegal to ever utter it (except for purposes of my explanation).
If you don't want to do something you simply say "Not me" OR "Dibs on not".
"Shotty Not" is just another prime example of Australia bending the english language over the counter and aiming for penentration.
now... don't make me slap you like a bitch
P.S I think that dude in the photo is now one of my heros