Just to make sure that I'm understanding you right, by Z-314.5 you mean what we know as Z315?
No. I am not measuring time to the nearest frame, but to the nearest tenth of a frame. Each tenth of a frame is roughly 5 milliseconds long.
z-312.0 is when the shutter opens for frame 312. Roughly 27 milliseconds later the shutter closes at z-312.5. Frame z-312 shows images from time z-312.0 through z-312.5.
I believe the bullet struck at z-312.5, or there abouts, right when the shutter closed or very shortly after. Below is a possible, approximate, time scale. Basically, my best very rough estimate:
Vis Time Time
able in in
sec. Zap Event
---- ----- ------- ---------------
0.000 z-312.5 Shutter closes.
0.000 z-312.5 Bullet impact.
0.001 z-312.5 Bullet leaves head,
momentum has been deposited into head,
the head starts moving at a steady speed forward.
0.027 z-313.0 Shutter opens.
vis 0.027 z-313.0 Head is now 1 inch in front of z-312 position, same steady speed
vis 0.055 z-313.5 Head is now 2 inches in front of z-312 position, same steady speed
0.055 z-313.5 Shutter closes.
0.077 z-314.0 Shutter opens.
vis 0.082 z-314.1 Neuromuscular spasm commences,
vis Head starts moving back, initially slowly.
vis 0.110 z-314.5 Head is now 1.5 inches ahead of 312.5 position.
0.110 z-314.5 Shutter closes.
“vis” refers to events that may be visible, because the shutter was open.
Again, this is not to represent exact locations. I don’t have the timing down to the nearest 5 milliseconds. Just a possible scenario consistent with what the Zapruder film shows and William Hoffman’s data.
In this scenario, the neuromuscular spasm started 82 milliseconds after the bullet impact. The real time of reaction could be a little more, or less.