According to Official Guinness Records,
The world's smallest pistol has a length of 5.5 cm (2.1 in), a height of 3.5 cm (1.3 in) and weighs 26 g (0.91 oz). It has a calibre of 0.68 mm and can be fired at a range of 5 m (16 ft). It is a scaled-down version of the Luger P-08 hand pistol, made of steel, plastic and rubber grips. The pistol was built in 2002 by Carlos Alberto Balbiani (Argentina) of Zapala, Neuquén, Argentina. The gun barrel measures 45.2 mm (0.17 in). It fires bullets (the balls of ball point pens!) measuring 0.58 mm (0.02 in) in diameter. It is non-automatic and uses gunpowder. NB: do not convert gun calibres.
For a complete list of 2002 records, please visit 2002 Guinness Records in Argentina.