The A-10 was designed around the GAU-8 Avenger, a 30 mm rotary cannon that is the airplane's primary armament and the heaviest such automatic cannon mounted on an aircraft. The A-10's airframe was designed for survivability, with measures such as 1,200 pounds (540 kg) of armor for protection of the cockpit and aircraft systems that enables the aircraft to continue flying after taking significant damage. The A-10A single-seat variant was the only version built, though one A-10A was converted to the A-10B twin-seat version. In 2005, a program was begun to upgrade A-10A aircraft to the A-10C configuration. The A-10's official name comes from the Republic P-47 Thunderbolt of World War II, a fighter that was particularly effective at close air support. The A-10 is more commonly known by its nicknames "Warthog" or "Hog". It also has a secondary mission, where it provides airborne forward air control, directing other aircraft in attacks on ground targets. Aircraft used primarily in this role are designated OA-10. With a variety of upgrades and wing replacements, the A-10's service life may be extended to 2028.