two-inch long pistol is the world 's smallest fully-functioning revolver .
tiny gun fires purpose-made bullets that are just 2.34 mm in diameter .
weapon was created by paul erard in the swiss city of la chaux-de-fonds .
each gun costs a staggering £ 300,00 to make - but is on sale for just £ 4,500 .
but new owners will require gun and export licences before they can buy it .