9mm pistol carried by a fellow emc-er

Its a number-

Different than a 9mm in a few ways.

-9s are fired sideways, rarely hitting anything. Used by street gangs.
-9mms are fired properly and hit where they are fired most of the time. Used by human beings.

The most basic inference of 9 is Death, or with the other angle Rebirth, like a phoenix.
The simplest usage of 9 is at The End of every digited set of natural numbers
Example: 1-9, 10-99, 100-999, 1000-9999, 10000-99999, etc, and thus the analogy with Death.
But as there is always a new bigger set after 9, it is also similar to Rebirth.

a pistol with a caliber of 9mm

When teens are texting they will use 9 to tell that a parent is watching or nearby

Emoticon used to express one's tongue out. Similar to =P.

Can also mean that something is delicious