The secret service was just starting to use AR-15 pattern rifles, chambered in .223.

You can see an agent with an AR-15 falling over when the motorcade accelerated, around the time JFK’s head exploded.

The only wound that jfk had that had bullet fragmentation was the headshot, despite the .30 caliber carcano rounds that Oswald was firing being very capable of punching straight through a head, and in fact passed cleanly through jfk’s spine and ribs without deforming or splintering.

The Autopsy report mentions how the bullet entered from the occipital lobe at the back of the skull, and fragmented, causing an exit wound by the ear.

.223 rounds commonly splinter when impacting bones, as they are fairly small and don’t maintain much momentum when they hit hard surfaces.