I watched that documentary also. It was by far the most plausible of all the theories.
Apparently, the Secret Service partied till just before dawn in Dallas the night before. That part is fact. Witnesses in the bar and hotel confirmed it.
The next morning a few could not perform their duties so one of the hung over officers handed their rifle to a secret service motor pool guy. He was riding inside the car just behind Kennedy.
According to the theory, The guy that shot JFK in the back of the head was a motor pool guy. This is 1 year before the M16 was issued to the military. And 6 years before the army made it the standard weapon. It’s a prototype and was given to the SS shortly before Dallas.
The first shots go off the Secret Service motor pool guy stands up just a few yards behind the car Kennedy was in. He was very close. I think they even showed a picture of trying to get Oswald‘s bullet design to fit in the hole in the back of Kennedys skull. It was too large in diameter.
It is very plausible. But is that what happened ? Who knows. Russia, the Mob, Cuba or all of that and an accidental discharge.
The second photo is of the motorcade speeding away after all the shots
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