The long-standing tradition of 'calling shotgun' when entering a vehicle goes back to the old west and the days of the stagecoach. Riding shotgun was a position of danger, being the man next to the driver who carried a shotgun to ward off bandits. This person was the primary target of anyone who wanted to attack the stagecoach.
Today the position is slightly less precarious, but murder may still be attempted in some families where one sibling manages to think of calling it first for too many days in a row.
In a family with a large number of kids, we have solved the problem by calling multiple guns. Shotgun is still the traditional position of being next to the driver. 'Squirt gun' is the driver's position. 'Machine Gun' is the seat behind the driver. Then there is the 'mini gun,' the 'gatling gun,' the 'water gun,' the 'air gun,' the 'potato gun,' and the 'marshmallow gun.' Last and certainly not least there is the 'pop gun.'
For some reason the kids seem pleased with their roles of such titles, but I fear that soon the names alone will be insufficient and actual weapons are going to be needed. I just hope marshmallows and potato bits don't start spattering against the windshield while I'm driving to say nothing of airsoft pellets or lead.