Far, far less than that. I regularly shoot targets at 100 yards with a pistol, though granted that's a stationary target. On the other hand, a plane coming in towards you is damned near a stationary target so while (if it happened), it probably involved both luck and skill I don't see it as being far beyond shooting skeet with a handgun which lots of people do. Remember too, if the story happened as told, he had four shots spread out over several hundred yards as the plane approached. Time to mentally adjust - hold higher, bang, lead him a bit, bang, not so much lead, bang, hold lower, bang... As an ex-coastie we had/have lots of guys that can put a slug through a go-fast motor from a vibrating helo while the go-fast is jumping through chop at high speed. Now, that's with a rifle, but that's not even consider luck - he's supposed to be able to do that, every time, on the first shot, and they do. Anyway, I have no reason to doubt the story is true, though I have no way of proving it's true. I do think it's entirely plausible.