What is Cameron meaning by saying "I call 9 millimeter"?

"Calling shotgun" is laying claim to the front passenger seat.

I saw that line as Cameron A) Not knowing what "call shotgun" meant, and B) Thinking John was saying "I want to have the shotgun" literally (e.g., take a literal shotgun on the "field trip"), and stating her preference for a 9mm handgun instead.

If it were much later in the series you might almost think Cameron was making a joke, punning on the double-meaning (and still saying she'd prefer a 9mm handgun), but certainly not in Queen's Gambit and I'm not sure she ever quite got that far before the series was cut.

"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.

From Wikipedia:

"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.

Per this wiki article:

The expression "riding shotgun" is derived from "shotgun messenger", a colloquial term for "express messenger", in the days of stagecoach travel the person in the position next to the driver. However, apparently the phrase "riding shotgun" was not coined until 1919.[2] It was later used in print and especially film depiction of stagecoaches and wagons in the Old West in danger of being robbed or attacked by bandits.

Meaning the person sitting next to the driver on the stagecoach would literally brandish a shotgun to protect the stagecoach from robbery, typically as they would be carrying a strongbox.

The character said 'I call 9mm', because the audience would see this as a play on the origin of this phrase.

According to IGFDB, the Glock 17 is the most commonly used gun on the show. It uses 9mm ammo, as do many of the other handguns.

So this is a brick joke, she's literally calling dibs on the hand gun. It's also an example of http://tvtropes.org/pmwiki/pmwiki.php/Main/BluntMetaphorsTrauma as Cameron is a cultural tourist on 2000's slang. She doesn't know the figurative meaning of the phrase as John is using it, likely thinking he called dibs on using literal shotguns.

It sounds more like something like this:

Sarah: Field trip. [with not much enthusiasm as "we're going to adventure"]

John : I call shotgun. [annoyed/sarcastic and as saying "gee... what adventure... I'll need a shotgun"]

Cameron: I call 9 millimeter. [slightly surprised "what are they talking about!?... I'll take that gun, youneverknow"]