Roy Kellerman, who rode in the front seat of the presidential limousine is by any standard, a five star witnesses - first, because he was a Secret Service agent whose job was, to keep and eye and an ear out for trouble.
Also, he was totally consistent with most of the other witnesses, who heard exactly the same spacing between the shots that he did.
Even more importantly, he was visible in the Zapruder film, so we can easily match up his testimony, with his actions. To put it another way, Kellerman did more than just tell us when shots were fired, he SHOWED us.
Kellerman stated that the time between the first noise/shot he heard and the second, was about 5 seconds.
"Mr. SPECTER. Was there any timespan which you could discern between the first and second shots and what you have described as the flurry?
Mr. KELLERMAN. I will estimate 5 seconds, if that. "
He described the final shots like this, "a flurry of shells come into the car" and "..it was like a double bang--bang, bang."
Even more convincing than his testimony, are his visible actions in the Zapruder film. During the attack, he ducked only twice - within a tiny fraction of a second following the shot at 285 and again, almost immediately following the headshot at 313. This animation makes that very clear (will not run properly until it loads and runs a second time).
http://jfkhistory.com/duckstwice.gif
And his reactions were simultaneous with reactions by Mrs. Kennedy and Mrs. Connally who dropped their heads at the same instant he did.
http://jfkhistory.com/angles285.jpg
The three of them reacted simultaneously with Bill greer, who began to spin around so rapidly that some people thought his turn was humanly impossible. It was while he was spinning that in his panic, he accidentally lifted his foot from the gas, slowing the limousine.
More corroboration comes from Dr. Luis Alvarez, who concluded that both Greer and Abraham Zapruder reacted to a loud and startling noise at precisely, frame 285. He said that Zapruder reacted at frames 290-291. The limo passengers and Greer, began to react at 290-292.
Watch that first animation again. Like Kellerman who ducked twice, Greer spun around from rear to front in perfect unison with Kellerman and the two ladies, as they dropped their heads.
In addition to ducking, Kellerman also exhibited other reactions which are textbook examples of startle responses to a loud noise. As he dropped his head, he raised his hand to shield his left ear, and hunched his shoulders upward and forward - exactly as described by Landis and Hunt, in their universally accepted textbooks.
http://jfkhistory.com/kellerman2.gif
As we go through the other visible, surviving witnesses in the limo, we will find almost perfect consistency among them. Each, heard only one early shot and were oblivious to the shot at frame 223, even Governor Connally who was hit by it. And with only the exception of Gov. Connally, who was about to pass out then, each described two shots which were fired at the end of the attack, or well after frame 223.
That doesn't mean there was no shot at 223; there obviously was. But it was not heard by the large majority of witnesses, as the Warren Commission confirmed, or by anyone in the limousine. Oswald's rifle was proven to generate an ear shattering, 130 decibels at ground level. The idea that one of his shots went unnoticed is beyond ludicrous.
At the very least, that shot was not fired by Oswald.
One last point to cover a loose end - one might wonder how we can be sure that the solitary early shot that was audible, was not the one at 223, rather than the one just prior to that, probably circa 150-160.
Mrs. Kennedy, SA George Hickey and SA Paul Landis, all stated that they turned to their right, in reaction the only early shot that they heard. In the Zapruder film, we can see each of them turning exactly as they described, well before frame 223. Ergo, the earlier shot was audible to them, but not the one at 223. Governor Connally reported exactly the same thing. He heard the earlier shot, but not the one at 223.
One might argue that someone heard the opposite, that 223 was audible to them, and 150-160 was not, but that seems highly improbable and even if it were true, it wouldn't change the fact that none of the early shots came from a high powered rifle.