In Altgens 6 we see Agents Landis, Ready and Hickey looking over their right shoulders towards the TSBD, presumably in response to the sound of gunfire:
Landis - "I heard what sounded like the report of a high-powered rifle from behind me, over my right shoulder...", "My first glance was at the President, as I was practically looking in his direction anyway...", "I immediately returned my gaze, over my right shoulder."
Ready - "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."
Hickey - "I heard what seemed to me that a firecracker exploded to the right and rear. I stood partially up and turned to the rear to see if I could observe anything. "
Each agent describes their immediate reactions to hearing the first shot, turning to look over their right shoulders looking towards where they felt the sound came from. This is exactly what we see in Altgens 6. However, when we take a closer look at Zapruder we see no meaningful reaction from them (Hickey looks briefly over the side of the car but then returns to his original position). The partial footage of the Z-film below focuses on the follow-up car. It runs from z133 to z207. At no point do we see any meaningful reaction from the agents mentioned above:
A first shot in the z130's or z160's is ruled out by this evidence.
Even a shot around z190 seems unlikely.