Porkie Mason. Rosemary was still running when she said she heard the first shot. Must take more than a frame to come to a stop. Maybe not in your world.
Jack Ready lifts his right hand off the front hand-hold and starts to turn his body to the right at z199. By z207 his right arm is down by his side.
All you're talking about is Ready having to release his right hand if he's to look more full rearward. It's like with Rosemary: you think she can do everything (hear the shot, process it, decide how to react) in one frame.
He said he turned around to his right in response to the first shot.
"At about Z164, Ready turned to face forward again, then at Z177
snapped his head to the right, and by Z255 had turned sharply to
his right-rear (as seen in a photograph by James Altgens)."
Dale K. Myers thinks Agent Ready reacted to the first shot earlier. Some clear frames ...
Agent Ready is standing camera-left on the front of the "Queen Mary's" followup-car running board.
"At about 12:30 p.m. we began the approach to the Thornton Freeway
traveling about 20-25 MPH in a slight incline. I was about 25-30 feet
from President Kennedy who was located in the right rear seat. I heard
what appeared to be fire crackers going off from my position. I immediately
turned to my right rear trying to locate the source but was not able to
determine the exact location."
To think Ready could turn
fully to his right rear "immediately" in a literal sense is not realistic, given his precarious perch on the running board. Ready describes "fire crackers" (plural) before he completed his full turn, which could mean he didn't get turned fully back until after hearing a second shot (ie: the second shot I place in the early Z220s works better than your nonsensical Z271 second shot). Or Ready could have meant he heard more than one shot in a general way and that he began to turn his head on hearing the first shot.