Its primary armament was eight .50-caliber machine guns and in the fighter-bomber ground-attack role it could carry five-inch rockets or a bomb load of 2,500 pounds (1,103 kg). When fully loaded, the P-47 weighed up to eight tons, making it one of the heaviest fighters of the war. The P-47 was designed around the powerful Pratt & Whitney R-2800 Double Wasp engine, which was also used by two U.S. Navy/U.S. Marine Corps fighters, the Grumman F6F Hellcat and the Vought F4U Corsair.