No. What was believed to be a silver gun in early, poor
quality bootlegs of the Zapruder film happens to be a reflection
off the forehead and hair of Agent Kellerman sitting in the front
passenger seat. Greer's hands can be seen on the steering
wheel, making it impossible for him to have done the shooting.
In Zapruder frame 312, taken just a fraction of a second before the fatal head shot, we can see Greer looking over his shoulder towards the back of the limousine. For him to have shot the President, he would have had to discharge the weapon no more than a foot or two from the Governor and Mrs. Connally. The bullet would have whizzed by their faces -- there is no possibility that they would not have noticed this shot.
Unless Secret Service Agent Kellerman was in on the plot, he would have surely tried to stop Greer from shooting the President. That was his job. But he didn't. And if Kellerman was in on the plot, why didn't he fire the shot instead of the man driving the limo?? It certainly would have made more sense.
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Is the Rifle in the Backyard Photo the same one found in the Depository?