Tell us what's going on in Zap
Everyone just continues to ignore Z329/330 where the real head shot comes in and you can see an unmistakable red smear over the President's head, his actual slumping right after and Jacqueline's instantaneous reaction to what has just taken place and wanting to leave the car through the back!
I would call that frontal shot absolutely undeniable given the driver Greer's head going to the corner of the windshield and Kellerman ducking out simultaneously. If Zapruder shook his camera, it was in reaction to what he just saw and heard - not Z313 however which has no explanation other than a very slight frontal head movement of the POTUS!