an alternate term for "calling shotgun." The person who calls "shotgun roundtrip," gets to ride in shotgun(aka the passenger seat) to and from the particular destination.
A group of friends are walking to the car to get ice cream..
Jamie: Shotgun Roundtrip!
Ang: Aw man, I wanted to ride shotgun on the way home!
Jamie: too late!
(Now Jamie gets to ride in the shotgun(aka the passenger seat) to and from the ice cream shop.