According to Official Guinness Records,
The most powerful mobile phone gun is a 0.22 calibre (0.22 in) pistol, disguised as a cell phone, which can fire a (close-range) lethal round of four bullets when numbers 5,6,7,8 are pressed in quick succession. It is believed that these phone-guns are made in Croatia, and have already been seized in drug raids across Europe in 2003.
NB: do not convert gun calibres.
For a complete list of Unknown date records, please visit Unknown date Guinness Records in Croatia.