The F-4E came into production as an improved version of the F-4C/D and was to be the most advanced model of the Phantom series. After elongating the fighter's nose and installing a more compact radar, the solid-state Westinghouse AN/APQ-120, the installation of a nose-mounted gun became possible and a 20mm M61A1 6 barrel Vulcan gun pod was employed. The Vulcan 20mm cannon, with its ammunition drum of approximately 640 rounds, was capable of firing 4000-6000 rounds a minute and proved reliable in air-to-ground and close quarters aerial combat. F-4E made its first flight in June 1967 and entered the rising conflict of the Vietnam War in November 1968 when the first F-4E's reached Korat in Thailand and joined up with the 469th Tactical Fighter Squadron. By the time of the cease-fire of 1973 the F-4E was credited with a total of 21 MiG kills, 5 brought down by gunfire alone.