Scooter’s HIPEG, from the man who brought you the Spruce Goose!
Here we see what was then dubbed the Mark 11, 20mm Aircraft Gun, in the Mark 1 POD, attached to the centerline bomb rack of a Douglas A4D-2 Skyhawk aircraft, April 14, 1958. The system was known in development by Hughes as the HIPEG.
“The U.S. Navy today unveiled a new pod-mounted weapon, a 20mm aircraft gun capable of firing 4,000 rounds per minute. This gun, called the Mark 11, was shown to Naval Aviators and representatives of the press at the Third Annual Naval Air Weapons Meet, held at the U.S. Naval Auxiliary Air Station, El Centro, California. This new weapon, which will offer a significant contribution to Naval Air Attack Capability, is carried and fired in an external pod which is fitted to the bomb rack of carrier-based aircraft. Its primary application is in an air-to-ground attack, where its controlled variable rate of fire makes it extremely effective. Ease of rearming, replacement of the gun, and maintenance are notable features which add to the practicability of the gun. Rear Admiral Paul D. Stroop, Chief of the Bureau of Ordnance, indicated that with the Mark 11 gun and POD meeting the firepower requirements of future attack aircraft, there will be a gain in aircraft structural simplicity since there would be no need for internal fixed guns. Mr. Frank Markquaret, a Naval Ordnance Engineer in the Bureau of Ordnance, conceived the Mark 11 gun and POD. It was developed for the U.S. Navy by Flier Industrialist, Howard Hughes. The Mark 11 is presently undergoing an evaluation at the Naval Aviation Ordnance Test Station, Chincoteague, Virginia, and is expected to be operational in 1959.”
According to a May 1962 report, 17 test pods cycled 16,000 rounds of 20mm ammo with a total of 29 stoppages or about 550-rounds on average between stoppages.
As reported by Popular Mechanics in 1963, three such pods could be added to a Navy jet to triple its gunfire available to somewhere around 12,000 rounds of 20mm per minute. It should be noted that at the time the A-4 mounted two Colt Mk 12 cannons (U.S.-made Hispano HS 404s), one in each wing root, with 100 rounds per gun.
Adopted as the Mk 4 Mod 0, some 1,200 of these pods were produced and served on Navy and Marine A-4s, F-4, and the OV-10 Bronco, primarily seeing active service in Vietnam for close air support missions.