Simple request. Find somewhere to view the Zapruder film. Find one of the enhanced-type versions where they zoom in on the head area, and you can view it at normal speed and at slower speeds. Hopfully, you can find the clearest copy available. Some copies are second- or more generation.
*Then*, ask yourself, where did the last shot come from, and does it look to you that his head is thrown back due to "muscle tightening".
However, please do likewise. My point about the gallon of water is that the physics of a high speed projectile rupturing a liquid filled vessel is very complex - certainly beyond my meager ability to explain. My own observation in watching a shooter (usually my boys) blast such a container is that shock waves actually propel the vessel towards the shooter. Thanks for your suggestion....I will follow-up.