I guess the Z-313 bullet was travelling about 1900 feet-per-second when its rounded point struck JFK's skull, causing both of them to break apart,
The salient point ...."I GUESS"..... And obviously your guess isn't based on knowledge of ballistics...... Because the idea that a human skull would cause a COPPER jacket on a FMJ bullet to break apart is absurd.
Copper has a high tensile strength and the COPPER jacket would not have shattered.
'the idea that a human skull would cause a COPPER jacket on a FMJ bullet to break apart is absurd'The idea that the human skull
wouldn't cause an FMJ bullet to break apart is absurd
'obviously your guess isn't based on knowledge of ballistics'Neither is yours
These are the guys who have knowledge of ballistics:
Gunshot wound characteristics
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5807523/[EXCERPTS]
2. Entrance wound bone damage (After DiMaio [8])
The bone of the cranial vault is made up of outer and inner cortical tables joined by thin cancellous bone (the Diploë) [12, p673]. The ‘typical’ appearance of an entry wound is that of a ‘broadening cone’ [12, p674] or crater [6, p261]. This is described as
‘internal beveling’ [8]. In a review of the skeletal remains of 21 gunshot victims, Quatrehomme and İșcan [13] found internal beveling in the bone entry wounds of 20 skulls but noted external beveling in one.
4. Additional fractures (after Karger [9])
The bony injury seen may be complicated by further fractures. Karger [9, p151] describes how secondary radial fractures are induced by the bullet’s impact and originate at the entry and exit sites.
Karger also describes how the brain is vulnerable to cavitation [9, p149] but the intact skull does not allow expansion, resulting in high pressures within the cranial cavity. If the overpressure exceeds the skull’s capacity to elastically extend, indirect concentric fractures result. Sufficiently high pressures will result in fractures combining to produce an ‘explosive’ type of injury [6, 9, 11, 14].
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Internal bevelling/HSCA

The Medical Evidence
https://mcadams.posc.mu.edu/medical.htm[EXCERPT]
When a bullet penetrates the skull bone, it will leave a small hole on the side from which it enters, and a larger dished-out crater on the side that it exits. The existence of beveling of the bone of Kennedy’s skull allowed the autopsists — and later panels of forensic pathologists — to establish that the bullet that hit Kennedy in the head entered from behind, with at least one large fragment exiting toward the front. See JFK Exhibit F-61, from the House Select Committee on Assassinations.