From History.net (https://www.historynet.com/gunfighting-north-vietnam.htm)
“A brilliant sky greeted Major Durward Priester of the 366th Tactical Fighter Wing (TFW) as he led a flight of four McDonnell F-4C Phantom II jet fighters on a combat air patrol (CAP) mission over North Vietnam on June 5, 1967. Flying at 17,000 feet with his backseater, Captain John Pankhurst, Priester could easily keep watch over the Republic F-105D Thunderchief fighter bombers being shepherded toward their target by the F-4C pilots. It was the Phantoms’ duty to protect the “Thuds” from enemy interceptors that might be lurking in the area. The CAP aircraft flew above and behind the strike planes in order to have maximum visibility as they screened the sky and terrain for any indication of North Vietnamese fighter activity.
As the strike aircraft approached their target, Priester spotted two North Vietnamese MiG-17 Fresco aircraft taking off from Phuc Yen airfield. The Soviet-built single-seat jet fighter had proven to be a nimble aircraft capable of downing F-4s and F-105s when flown by skillful pilots. Priester watched as the MiGs maintained a low altitude and turned toward sanctuary across the border into China. U.S. rules of engagement forbade American fighters to cross the border in pursuit of North Vietnamese aircraft.
Priester figured that these two pilots were probably inexperienced and were simply moving their aircraft across the border. However, a third MiG rose behind the first two and quickly began climbing to a higher altitude. In the early years of the conflict, North Vietnamese pilots frequently flew a formation known as the “stack three,” in which three planes were staggered in altitude and separated by approximately one mile; but these MiG-17s were flying a modified stack three formation, with the third plane carrying the most experienced pilot and rising to protect the other aircraft making haste toward China. In fact, the other two MiG pilots were in such a hurry to escape, they flew through the falling bombs now being dropped by the F-105s over their target.
Priester quickly decreased his altitude and maneuvered toward the 8,000-foot altitude of the third MiG. Normally, F-4Cs carried an assortment of air-to-air missiles, but lacked automatic weapons such as cannon for close range fighting. Priester’s aircraft, however, was armed with a new field-modified innovation—an SUU- 16 gun pod carrying a 20mm cannon attached to the F-4C’s bottom centerline section. Priester recalled, “I pulled up and in trail with the number three MiG, as the MiG executed a hard right turn. I fired a short burst but saw no evidence of the 20mm rounds hitting the MiG.”
The gun sight mounted in his plane had burned out and Priester figured that he was probably firing in front of the MiG. “I relaxed and fired a second short burst and I could see the rounds hitting behind the MiG’s canopy. Although I did not see smoke, a big ball of fire rolled out of the MiG’s tail. The MiG banked hard into a 30 degree dive and crashed into a military barracks area.”
The MiG pilot did not eject from his aircraft. Priester reported that the entire engagement lasted approximately 45 seconds and he expended only 202 rounds of 20mm ammunition to bring down the MiG. The kill made by Priester and Pankhurst that afternoon was the fourth in 1967 to be made by an F-4C carrying the field-modified SUU-16 gun pod and helped pave the way for the United States to reintroduce cannons to all of its future fighters.”