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.
Universallyrecognised as the best number in the world, used to express something or someone's brilliance, but can also be used to describe a complete twat
Paul and I went to Paris last week, the Eiffel tower is a massive 9 in real life.
FFS have you seen that guy Mac over theretrying to be cool with his balding head, he's such a massive fucking nine.