I've seen that done on TV in an investigation of the Kennedy assassintation. However, it appears even simpler than that. The frame by frame analysis of the Zapruder film shows that Kennedy's head clearly went rapidly forward and down for one frame after he was shot. It then went back slower over the next two frames. The reason his head ended up back was because his chin bounced off his chest. You can see the same effect in head-on auto accidents where the driver's head ends up breaking the headrest behind him.
Anyone who sees the Zapruder film and concludes that JFK was shot from the front simply wants to ignore the physics and believe some conspiracy theory.
Amen! Also, I would like to know what kind of weapon was supposedly used from the "grassy knoll" that emitted a puff of smoke as the conspiracy theorists claim. Must have been some old gunpowder powered muzzle loader since all propellants I know of today are virtually if not completely "smokeless".
Too much of Kennedy’s brain ends up on the trunk of the limo for you to convince me. Jackie crawls out of her seat, reaches back there to retrieve the chunks. It is ballistically impossible for that much mass to end up that far to the rear, given that the head and body are clearly moving forward at that point.