Watch 'JFK; The Smoking Gun', which pins the head shot on Secret Service agent George Hickey. Here he's holding the AR-15 that he allegedly accidentally fired into the back of Kennedy's skull (when vehicle lurched forward, causing Hickey's finger to pull the trigger) with a frangible round that exited out the right top/front of Kennedy's skull.
I agree with this theory. It sure seems as though the deadly head shot exited out the top/front of Kennedy's head and that an 'exploding' bullet was used, unlike the type of ammo Oswald used which was found to be intact and barely damaged.
This head shot lines up perfectly as Kennedy's head is bowed down after have already been shot through the neck by Oswald. At least 11 people on the ground testified to smelling gun smoke at street level after Kennedy was hit. Oswald was 6 stories up and over 250 ft. BEHIND these vehicles. Do the math.