The Fart Gun is a weapon that was gated off as pre-order bullshit for the Russian version of Postal III. Similar to the Shop Vac, the primary fire dispenses a foul gas that immediately knocks out any enemy in the vicinity. While it's ammo capacity is unlimited, it does need to be reloaded, which is done by holding down the alt-fire button.
The Fart Gun is unlocked at the start of Diseased Cat Roundup in Story Mode, provided you have its files installed.
The Fart Gun was never properly released outside of Russia. Likewise, it isn't available for purchase anywhere. Fortunately, the weapon has been ripped by Mike Rosenberg and can now be ported to international versions of the game.