I know what you're saying Charles but it's not the same thing. In Altgens 6 Landis, Ready and Hickey are twisted around looking behind them, it's an extreme, co-ordinated reaction by all three and they each testify it is a specific reaction to the sound of the first shot.
It's not just head turns, these are to be expected and should not be used to support the identification of a gunshot.
Altgens 6 was taken at approximately Z255. This is about 1.75 seconds after Z223 (when you say that you believe that the first shot occurred). If your hypothesis is correct, then it means that the following conversation and actions took place in that 1.75 seconds:
“Landis elaborated that he looked around, “observed nothing unusual,” and “began to think to think the sound had been that of a firecracker.” Landis recalled that Agent John Ready, in front of Landis on the running board, asked him “What was it? A firecracker?” Landis answered, “I don’t know; I don’t see any smoke.” Landis then thought that the sound might have been a tire blowout; he looked at the right front tire of the president’s limousine and saw that it was intact. He could not see the right rear tire from his position. Landis then saw Kennedy’s head “split open” and realized that the first loud sound had been a gunshot, just as he initially thought."
No way, Jose’!!!