Jerry
“I didn't claim it won't crack the skull, or bore through it. I'm assuming it would penetrate the skull bone, but, being frangible, it would leave behind at the impact site a considerable amount of its mass.”
What do we know about the hole, it appears to be a clean hole so the bullet did pass through the skull without any “explosion” at entry. So then the bullet travels some distance and explodes, how far we don’t know. We also know that the right of JFK’s head is blown out, shown in Zapruders film.
I can see a FMJ making it through the skull and leaving behind a clean hole. "Mortal Error" reported that Donahue knew the 6.5mm bullet "would have fractured mechanically, that is broken up as a result of the physical impact with the skull." We don't know the exact mechanism involved, but there is some speculation that the 6.5mm bullet, hot from being fired, would have encountered significant friction during its passage through the bone and fractured along its middle length. That portion of the lead core could liquify and send "cast-off" fragments throughout the brain. When the skull exploded (from the temporary cavity and impact on the inside of the skull) some metal fragments could end up in the scalp itself.
"Mortal Error" includes a very similar description of "cast-off" fragmentation with regard to the .233 round:
"The missile's tumbling motion, combined with its enormous centrifugal
velocity, then shredded the already ruptured jacket and flung the now-
molten lead core outward in a circular pattern near the front of the brain."
The Haags fired a few 6.5mm rounds into asphalt and discovered the bullets disintegrated into minute bits.
"In this case, a bullet traveling approximately 2100 f/s (1,432 miles per hour)
comes to a complete stop in about 1 inch of travel in asphalt. A rough
calculation of the decelerating forces acting on this bullet, expressed as
gravities (g), yields a value of 1.6 million g. The relatively soft copper jacket
disintegrates into minute fragments and the very soft lead core essentially
vaporizes."
“But whatever passed through Kennedy brain didn't arrive there nose-on and intact.”
No, it exploded as frangible rounds do, we just don’t know where it explodes.
"Illus. 16" shows the .223 round traveled without disintegration through a distance much greater than the brain before breaking up.
Moving at 3,000 ft/sec it could have traveled some distance. Of course it would have been slowed down by the impact but none the less, it was still traveling very fast.
“Great test, I suppose, if the working assumption is that a FMJ bullet will be intact and nose-on after passing through a bone. McLaren would love it; the AR-15 bullet would disintegrate anyway and the FMJ bullet would plow a straight path.”
Again, we know it will designate or explode at some point but we don’t know where.
There is an Australian study on-line that may or may not relate to the AR-15. It's called "Wound Ballistic Simulation of a Headshot with .223 Rem"
link. It refers to a head wound in the temporal bone. FWIW, here it is:
The Abstract:
"In the context of a crime scene, it became necessary to examine
whether a direct shot to the head with calibre .223 Rem. could
happen without creating an exit wound. A simplified head model
consisting of a bone-sphere filled with gelatine was compared with
a more realistic model considering the inner bone structure of the
human skull. It turned out that both models show significantly
different behaviour. While the bullet normally penetrates the
simplified model and causes an exit defect, it was found that bullet
fragmentation can happen in the more realistic model due to grazing
along inner bones. The fragments might not perforate the skin and
remain inside the skull without creating an exit defect."
Some excerpts:
"Experience from previous examinations is that a .223 Rem. bullet
fired into a 15 cm block of gelatine penetrates it, creates a large
temporary cavity and leaves an exit wound with a diameter multiple
times the bullet’s diameter. When shooting at a simplistic head model,
such as the one described above, this behaviour is confirmed: the
bullet enters through the Synbone®, passes through the gelatine
without fragmentation and exits through the Synbone® on the
opposite side."
"All bullets shot into the simplistic models hit the surrounding bone
head-on and penetrated without fragmentation, leaving an exit wound
that was afterwards widened by the temporary cavity that the bullet
caused. The surrounding bone-sphere was heavily fragmented. This was
observed both with skin substitute attached to the bone and without.
The .233 round went through a "simplistic head model" and did not suffer metal fragmentation.
"The five shots using the setting shown in Figure 1 [a more realistic skull
model] hit a flat Synbone® immediately after they left the muzzle in a
grazing manner with an angle comparable to the angle a self-inflicted
shot might graze along the temporal bone (see Figure 1). This setting
causes the bullet to yaw so it hits the block of gelatine with an enlarged
surface rather than with the top ahead (as in the case of the simplistic model).
The force acting on the enlarged surface causes the bullet to fragment
when it enters the block of gelatine and also subsequently.
Out of the five shots using the setting shown in Figure 1, one fragmented
in such a way that the biggest fragment created an exit wound on the
opposite side. Two shots could not be analysed because the trajectory
moved away from the camera position. The remaining two shots did not
show any exit wound because the specific energy of the heaviest fragment
(the steel tip of the bullet) was not high enough to perforate the skin simulant."
"If the bullet grazes a substantial bone it starts to yaw and it fragments.
The specific energy of the fragments is much lower than it is for the bullet
as a whole and might not be enough to perforate the bone and the skin at
the opposite side of the skull. The induced pressure still causes the skull to
burst but may not cause the skin to tear. Thus, the fragments remain inside
the head. The skull will show multiple fractures but its pieces could still be
held together by the enveloping skin. In this case there will be no exit wound."
