Yeah, what Art said. I think many neck shots, even when they don't actually contact the spinal column, impart enough enough shock to the surrounding tissue (including the spine) to disrupt things.
The most instantaneous kills I've witnessed were with a .243 believe it or not. I was using a 70 grain Barnes "X" loaded to max velocities and no matter where I hit them, they went down right there - I say that with the understanding that all those hits were good ones, chest shots with the occasional neck shot here and there.
I must have shot 12 or 15 deer with that load and all of them were "one shot stops". Prior to that I had been using 100 grain Partitions (both Federal Premiums and later, a handload) and in nearly every case the deer traveled a short distance before keeling over.
In the real world, both of those loads were just fine. But, I think that 70 grain X at about 3700-3800 fps just had a hell of a wallop and that velocity (since it was incorporated in a good bullet) was responsible for the instantaneous nature of the kills.
Side note - those X's at high velocities leave you to do a lot of scrubbing with copper solvent. No free lunches here.
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