This effect is familiar to others who have experience with high-powered projectiles. Especially with many types of rifle rounds, only a very little energy is transferred to the target in a direct linear fashion that creates momentum. This is a common effect in high-velocity collisions and generally not appreciated by laymen. The damage the bullet does to a soft target is typically from temporary cavitation, an effect observed in Kennedy's case, and which is particularly devastating for head shots because the cranium is not sufficiently elastic and evulses rather than rebounds in response to the cavitation pressure wave.
Axial transfer of energy does not dominate the physics here. Instead, energy is initially diverted transversely as a result of the fluid response, much the way a boat's wake moves laterally. Then other properties of fluid dynamics entrain (draws along) fluid material behind it. The bullet that struck Kennedy had a relatively high sectional density, thus decreasing the primary effect and increasing the cavitation and entrainment. The rearward component of the President's motion is the Newtonian response to the ejecta entrained by the bullet. The leftward component is the Newtonian response to the momentum of the evulsion resulting from the asymmetrically uncontained cavitation.
During the first half of the 20th century, the design of infantry rifles and their ammunition favored a high power, high velocity, high sectional density round. The infantryman was considered little different from a big-game hunter. He needed an rifle that was accurate and had plenty of penetrating power. This was the equipment Oswald was able to obtain privately.
Had Oswald chosen a "modern" 7.62 millimeter slug (especially a fragmentation bullet) and an assault rifle of the 1960s, the results might have been different. More of the energy would have been transferred in the primary bullet channel, and the supposed Newtonian reaction "back and to the left" would have more justification in physics. But the missile that hit Kennedy was slim, heavy, and moving very fast. The skull responded to the impact not by absorbing it elastically but by shattering. No momentum created there. The cerebral matter responded not by spongily absorbing it but by being deflected to the sides (pressurization, but little net momentum except where the already-weakened cranium failed) and then being entrained (lots of momentum). The Newtonian reaction to this phenomenon creates "back and to the left."
The conspiracy theorists generally first applied only pidgin physics to this problem. Hence the "back and to the left" observation in the Zapruder film necessitated a shooter in front of Kennedy and to his right, which suggested some location variously along the Grassy Knoll. (Only a very few locations within this area would have concealed a shooter, none of them especially conducive to the purported fatal shot from the front.) However, nowadays the pidgin physics have largely fallen by the wayside and conspiracy theorists now focus on trying to discredit the Zapruder film as a forgery.
WRT "Fragmentation" bullets.
I once became enamored with pre-fragmented bullets, specifically the Glaser Safety Slug in it's various calibers, both handgun and rifle. The advertising hype back in the late 1970's early 1980's was "Stopping power second only to the 180 grain JHP .44 Magnum...in a Nine Millimeter!"
I fell for it hook, line and sinker.
The rounds fed and functioned 100%, good to go there, and the accuracy was on par with standard defensive rounds, good there too. The only problem, which never came up in static testing, is that on living human targets that shoot first or shoot back there wasn't much terminal effect at all outside of a headshot - first hand experience otj by one of my friends had the round impacting and making a nasty surface wound, but the thing didn't penetrate deep enough to stop the target.
In the rifle calibers, and I had them in 7.62 NATO and 30/06, I then decided to get a quantity of ballistic gelatin to see if the rifle calibers would be similar or better. On BG, the rounds created a more or less bore diameter entrance and the gelatin had major displacement from the temporary cavity and the dispersion of the birdshot from the round, but the rounds didn't reach the desired 12" or more penetration for defensive purposes on a human target.
The rounds belong in a cartridge collection, not in a firearm intended for defensive use.
I could have sidestepped all the hype and drama by simply listening to my father, who said when I told him about the stuff before I knew the score - "There's no such thing as a magic bullet."
As to head wounds from the .30 caliber class of rifles, a solid hit within the ranges of the JFK shooting distances would not have left much, even using standard military ball ammo, but even headwounds aren't always an instant stoppper. I know of two individuals that have survived headshots with the former combloc 7.62 x 39 round, which isn't in the same class as .303 Brit, 7.62 x 54r, 7.62 NATO or 30/06, but you'd think rifle caliber rounds to the head would be garranteed fatal, but anything can happen.
