Halfway through Oliver Stone's ''JFK,'' one of Jim Garrison's detectives, sitting in the fifth floor of the Texas Book Depository with a Mannlicher-Carcano 6.5 millimeter carbine, leans out the window, dry-fires once, awkwardly throws the bolt, fires again and then repeats the process for a third dry shot.
''It can't be done,'' he says to Kevin Costner. ''No way he could fire three shots in 5.6 seconds.''
In fact the belief that it is impossible to fire three shots from that rifle in 5.6 seconds, as the Warren commission maintained, is the very heart of conspiracy culture: It is the rock upon which the church is built. All other filigrees of paranoia extend radially from it: the magic-bullet theory, which explains the maximum damage done in the minimal time span, the belief that other shooters were involved, the idea that Lee Harvey Oswald was a patsy, manipulated by figures from the intelligence community or organized crime, on up to the politically-correct industry that has reached its apotheosis in the Stone film and convinced 77 per cent of the American public that a conspiracy existed.
Amazing how some lies persist, isn't it?
And not only is the assertion patently false, knowledge of its falsehood has been in the public record since 1977, where it was placed by, among others, the Baltimore Sun Magazine.
In May of 1977, the Sun Magazine published a fascinating two-part series by a great old-time newspaper reporter named Ralph Reppert, entitled ''The Kennedy Assassination: A Different View.'' The story was based on extensive interviews Reppert held with a Towson gunsmith named Howard Donahue. Ralph has since died; Mr. Donahue continues to earn a living as a firearms consultant.
It was (and still is) Mr. Donahue's theory that in the frenzy of panic, John F. Kennedy had been accidentally shot in the head by one of his Secret Service bodyguards with a 5.56mm-16 rifle. Indeed, in the car behind Kennedy such a weapon was present, as a photo published in the Sun Magazine made clear.
The basis for Mr. Donahue's interpretation was that the bullet that hit Kennedy first and then Governor Connolly was largely intact -- as would a heavy, jacketed military 6.5mm round designed for penetration -- while the bullet that struck Kennedy in the head fragmented -- as would a light, high-velocity 5.56mm round. Mr. Donahue has other evidence to support his theory.
One may or may not agree with his thesis. But one may not disagree with the implications of a shooting exercise he took part in some years ago, an episode that initially interested him in the assassination. In 1967 was a volunteer shooter for a series of tests held by the H.P. White Ballistics Laboratory near Bel Air that turned out to be a re-enactment of the shooting problem of the Kennedy assassination for CBS news.
Thus it was that Mr. Donahue, a pretty fair country rifle shot but by no means a shooting champion, found himself 60 feet up in a jury-rigged wooden tower with a Mannlicher-Carcano, taking a bead through the four-power scope on a human silhouette moving before him at exactly the same angle and speed (11 miles an hour) that John Fitzgerald Kennedy had moved before Lee Harvey Oswald. How familiar was Mr. Donahue with the Mannlicher-Carcano? Not as familiar as Oswald. He'd first held it in his hands that morning.
Mr. Donahue jammed the rifle on his first try; on his second, he only got off two shots within 5.6 seconds, though both were hits.
''In my third series of three,'' he told Reppert, ''I fired a split-second after the target passed the first stake [marking the beginning of the time sequence, something that Lee Harvey Oswald didn't have to worry about]. With my right hand, I hit the bolt handle to eject. It was stuck again.
''Realizing how little time I had, I hit the bolt with all the force possible, ejecting the empty. I felt more than half my time had passed when I slammed the bolt forward to insert the second time, picked up the target in the cross hairs, and fired.
''Even while the rifle was in its slight recoil, I brought up my right hand, hit the bolt to eject, slammed it forward to load, picked up the target and squeezed off my final shot.
''By the time I ejected the last shell, took off the sling and stood up, the target had been returned and was being examined. My three shots were hits, within a 3-inch circle in the central head area. I heard a technician at the base of the tower call out: 'We've got a good one! Four-point-eight seconds from first to last shot.' ''
Even a second timing device measuring the event at 5.2 seconds still brought the shooting sequence to conclusion four-tenths of a second faster than Oswald's. Mr. Donahue hadn't simply matched Oswald, he had shot faster and better. And, just to make it more interesting, it was raining and the wind was blowing, swaying the tower.
Does this prove that Lee Harvey Oswald shot John Fitzgerald Kennedy? Of course not. But it does establish beyond a shadow of a doubt that any theory of the Kennedy assassination which extends from the supposition that it was physically impossible for a man to have fired that quickly and that accurately with that rifle is simply incorrect. It was possible and a good shot could bring it off with very little practice.
Of course those who believe in the conspiracy won't care. For them, the knowledge that Kennedy was murdered by a conspiracy, probably involving CIA, has taken on the force of religious faith; it occupies some zone beyond logic. In fact, for many of them, the lack of any physical evidence of a conspiracy is proof positive itself that a conspiracy exists.
But one Marylander squatting in a tower on a rainy afternoon proved that it could be done by a lone gunman.
Stephen Hunter is The Sun's film critic.