The first shot was described as "ear-splitting" and an "explosion."
By everyone who heard it?
The three agents would have spun around within a second of hearing it.
Even if they thought it was just another motorcycle backfire?
One must reasonably conclude from [the] evidence that the first shot did not occur as early z160, let alone earlier.
From “Estimating Occult Timing of Surprise Gunshot Sounds in Silent Film via Observed Start of Human Voluntary Reactions of Concern” by Brian Roselle and Kenneth Scearce in “Journal of the Association of Crime Scene Reconstruction”; 2020
First number = Author A’s observation; Second number = Author B’s observation; Third number = average of the two. Numbers represent Zapruder frames.
Roy Kellerman -- Begins leaning over and looking behind/down to the right at 148, 148, 148
George Hickey -- Begins leaning over to the left looking down in the direction of the rear tire or tire at 144, 143, 143.5
John Connally -- Begins a quick head turn left (followed by quickly looking back right) at 151, 149, 150
Jackie Kennedy -- Starts accelerated head turning left, before looking back right. (Similar to John Connally’s L-R head motion but starts slightly earlier & ends slightly later than his) at 145, 142, 143.5
President Kennedy -- Starts a quick look to the left at 144, 142, 143.5
Nellie Connally -- Begins a quick sweeping head turn to the right at 146, 144, 145
Rosemary Willis -- Begins a quick look away from the Presidential limo back towards the Texas School Book Depository at 141, 139, 140
Rhetorical question: What are the chances that all five passengers (i.e., non-drivers) in an automobile would randomly move their heads within one-third of a second of each other?