The 9mm is a gun appeared in the Xbox360 versions, PC and PS3 Alone in the Dark (2008).
You'll find the gun in the hotel at the beginning of the game. It will be mandatory in order to pick it up to shoot the lock of a locked door just ahead. Its capacity is 8 bullets and the firepower is low. Nevertheless it can be very handy if used intelligently. Pouring the liquid on the flammable bullets will make them become incendiary. The 9mm will automatically be abandoned once the .44 Magnum is recovered.