Not the definition of the "girth" of the barrel, but rather it's bore - the width of the hole that goes through the barrel. Also, not coincidentally, the diameter of the bullet - 9mm.
Refers to a pistol, usually a semi-automatic, ostensibly a 9mm.
Aside: The U.S. Army's standard pistol is a 9mm semi-automatic.
"I was about to pull out my nine and bust a cap."