9mm (or nine-millimetre, depending on your locale) is a general
term for a family of firearm ammunition. The value refers to the
diameter of the bullet itself; the casing may vary in size.
Typically, this caliber (for you Americans) ammunition is used
in handguns and submachine guns (machine pistols). There are a
variety of common subtypes of 9mm ammunition. Some include: The
most famous early adopter of the 9mm standard was Georg Luger,
in the magnificent pistol that bears his name. The Luger was
originally a 7.62 and 7.65mm weapon (it also even sported the
.45 ACP for a U.S. procurement trial series that resulted in the
U.S. purchasing the Browning M1911). In 1904, while still
attempting to garner large-scale contracts, Luger retooled the
gun to fire a larger bullet in order to allay concerns over the
stopping power of the weapon. He designed a cylindrical, round-
nosed shell, 9mm in diameter and 19mm in length. This version of
the weapon was introduced in 1903/1904, and the German Army and
Navy accepted it