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While a cameraman will involuntarily jiggle his camera in response to hearing a rifle shot, it is not the only thing that will cause a jiggle so additional observations need to be factored in. Since the distance from the muzzle to Zapruder's ears would have been a constant for all three shots, we should expect a similar lag time between the shot being fired and Zapruder's response. We can start with the most obvious, the head shot, which struck at Z313. That shot traveled 88 yards. Estimating the average velocity of the bullet over those 88 yards, it would have taken 0.132 seconds for the bullet to reach JFK's head. Each frame of the Z-film covers 0.055 seconds so that equates to slightly more than 2 frames of the Z-film which would indicate the bullet was probably fired at Z311. It could have been fired late in the exposure of Z310 or even during the brief gap between Z310 and Z311 when the film would not be exposed. That establishes that the bullet was fired in the Z310-311 window. Advancing forward, we find a significant blurring at Z318. If that blurring was caused by Zapruder's reaction to the gunshot, that establishes a lag time of between 7 and 8 frames between the firing of the shot and Zapruder's reaction. It would be nice if that number were and integer, but that's a bit unrealistic to expect that. We can work with the approximation.
With the lag time established at between 7 and 8 frames between the shot being fired and Zapruder's reaction we can move on to the second shot. We see a significant blurring of frame Z227. Using the 7 to 8 frame difference between the shot being fired and Zapruder's reaction, that tells us the shot was fired in the Z219-220 window. That shot would have traveled roughly 190 feet to reach JFK's back. Again using an estimated 2000 fps velocity, that means it would have struck roughly 1.75 seconds latter, so sometime in the Z221-222 window. I lean toward Z222 but since we are working with estimates Z221 is a possibility. Either way, in rapid succession we see JBC's jacket bulge at Z224 and the simultaneous upward arm movements by JFK and JBC at Z226. This is powerful evidence that both men were hit during this time frame.
Moving on to the first shot, JBC shows a visible reaction at Z164. We don't know what JBC's lag time was between hearing the shot and his reaction, but Z164 gives us a good starting point. If we work backwards from there, Z158 shows blurring and for years I thought that was when Zapruder reacted but had I looked back a few more frames, there is more pronounced blurring at Z155 which seems to be the more likely time of Zapruder's involuntary reaction. That would indicate a shot in the Z147-148 window.
Since we have no visual evidence of a bullet strike for the first shot, we have to recognize that the Z155 jiggle might have been caused by something other than a gunshot, but given its proximity to JBC's reaction, I think the preponderance of evidence leans to the first shot in the Z147-148 window. Some have argued that the first shot was fired was fired before Zapruder resumed filming. While I cannot logically rule that out, it would require us to believe JBC reacted very slowly to the sound of the first shot. Some have actually argued they can pinpoint a theoretical Zapruder frame had he been filming the entire time. I'm not sure how they could do that with no visual evidence but I'll leave it to them to explain how they could pinpoint such a shot.