Thanks for the inputs. This has been an question whenever the thought has cross my mind. I looked up the closure rate yesterday and it would have been abt 3600 feet per second. I must admit that no damage was seen on the aircraft, I went over the plane carefully, the pilot did not recall seeing any AA fire, which is why it's been this lingering question. I was 3 feet behind him. (F-4 back seat) The target was definitely a 37mm AA gun. AAA fire was often seen before but only this once from this angle. This was combat sortie maybe 125 or so.