Andrew Mason: Thanks for your collegial inquiry.
I have looked at the Z-film a thousand times, and frame by frame.
I am a layman, though I am not sure there is such a thing as an "expert" at looking at blurry home movies and divining the moment of impact of a bullet on a body. Each to his own.
Another bit of Mrs. Connally's testimony, again indicating JBC was pushed forward at the moment he was struck from behind by a large slug, that tunneled along a rib, meeting resistance, and pushing his body forward.

As I see it, JBC lunge or is pushed forward ~Z-295.
The LNT-SBT theorizing evolved in response to President LBJ's diktat that LHO be found a LN with no leads to Havana or Moscow (no nuke war please), and the WC's subsequent necessity in making the evidence line up with that premise.
There were three loud shots heard deep inside the TSBD on 11.22, and that indicates there was in fact a TSBD6 sniper who fired three shots. Prime suspect is LHO.
The problem is the shot cadence---the shots that strike JBC and JFK are too close together to have been fired by a lone gunsel armed with a single-shot bolt action rifle. The "bang....bang-bang" cadence. Dallas Sheriff Seymour Weitzman described the last two shots as nearly "simultaneous." A man with much experience in firearms.
There is the possibility of simultaneous shots heard as one shot on 11.22, or of someone firing using a silencer.