12,7mm is in fact not the worlds biggest bullet, cal. 700 Nitro Express is, although 12,7x99 is the worlds most powerful non-wildcatin terms of energy and speed. 0.7 Inches is 17,8 mm, folks! The 12,7x99mm was originally intended as a crossing between a rifle cartridge and a small cannon shell. It is used mainly in the .50 cal. Browning HMG, and in heavy sniper rifles, like the Barrett.