High Quality Content by WIKIPEDIA articles! The .500 Nitro Express is a .510 caliber (13 mm) big bore hunting cartridge. It was designed for use in single shot and double rifles. The cartridge is available in two lengths: a 3.00 in (76 mm) and a 3.25 in (83 mm) version. The cartridge was designed for use against large dangerous game species in Africa and India.