"Calling shotgun" is a way of making sure that you get to sit in the front seat of a car, similar to calling "bagsies" in order to secure something for yourself.
"More recently, the term [shotgun] has been applied to a game, usually played by groups of friends to determine who rides beside the driver in a car. Typically, this involves claiming the right to ride shotgun by being the first person to call out "shotgun"."
Cameron then calls "9 millimeter", another word that has connotations to weaponry, specifically guns. I would assume that "9 millimeter" is meant to trump "shotgun", meaning that Cameron would get the front seat.
Another interpretation of this line (courtesy of @NapoleonWilson) is that Cameron simply didn't know what calling shotgun meant, and as such simply named another weapon.
However, I feel pretty confident in saying that the lines inclusion in the show is intended as a reference to a line from this iconic scene from The Terminator.
The line "uzi 9 millimeter" quickly became a popular reference to this movie.