The Republic P-47 Thunderbolt, also known as the "Jug," was the biggest, heaviest, and most expensive fighter aircraft in history to be powered by a single reciprocating engine. It was one of the main United States Army Air Forces (USAAF) fighters of World War II, and also served with other Allied air forces. The P-47 was effective in air combat but proved especially adept at ground attack. It had eight .50-caliber machine guns, four per wing. When fully loaded the P-47 could weigh up to eight tons. A modern-day counterpart in that role, the A-10 Thunderbolt II, takes its name from the P-47.