Here’s a thread on the fragments from the fatal head shot with some good cites:
http://boards.straightdope.com/sdmb/showthread.php?threadid=162165
akrak01, I’ve seen a copy of the film that looked like the driver turned around and fired the fatal shot. I’ve seen other, better-quality versions that didn’t look like that. Just before the head shot, the driver turned around to look back. There was a glare from a the passenger side window on the front seat, which in that one copy, looked like something metallic followed by a flash.
However, I didn’t know that anyone actually took seriously this idea.
About 3 or 4 years ago, I was listening to the Dallas NPR station and they had on the lawyer who represented Abe Zapruder in 1963. He swore up and down that the footage that most of us associate with the term “Zapruder Film” is not what was brought into his office back in 1963. He says that the footage that we usually see has been ‘highly edited’.
By the way, the jet effect is just Conservation of Momentum in action. A bullet has a lot of kinetic energy, but not a lot of momenum to impart directly, especially if it passes through. Instead, if it blows out a chunk of what it hits, the momentum of the chunk that’s ejected has to be balanced by the main part of the hit object going in the opposite direction. In the case of Kennedy’s head, the bullet hit high and right of center, and the section that was blown out was close to 90 degrees away from the path of the bullet. The bullet itself wouldn’t have transferred much momentum, because the head didn’t stop the bullet, so the mass being ejected up and right would kick his head back and left (his head was tilted down and turned to the left). There’s also speculation that muscular spasms caused by the massive trauma pulled his torso back at the same time (it triggered all muscles to sieze, and the stronger back muscles won).