Mostly Murder and the 1940's shooting.
Mostly Murder is the fascinating biography of the work of Sir Sydney Sheldon. He was a forensic pathologist who worked in colonial Egypt during the First World War and moved to Edinburgh before the second. In one of his cases he describes a man who was shot in the back of the head while a passenger in a military car. The resulting wound and surrounding ballistics are very similar to Kennedy's right down to the confusion over the number of bullets fired (in this case only one).
He begins by describing his experiments with British .303 Second World War rifle bullets and his discoveries of their behaviour at short range. This was at a time when forensic ballistics were at their infancy. He discovered that for 2 to 3 hundred metres these high velocity bullets exhibited a certain degree of 'wobble'. The back end gyroscoped in a larger range than the front as the bullet spun, and that this wobble evened out over distance.
The man was shot at very close range with one of these .303 bullets. The bullet first passes through the army vehicle's back celluloid (plastic) window and hits the man in the chin from behind. The man later dies of infection and during the post-mortem Sheldon finds a clean-cut entry wound indicating the bullet was whole when it entered but created a 9 cm long exit wound which had the appearance of exploding outward (Kennedy's was 13 cm). The lower jaw was smashed into fragments; 'it appeared to have merely touched the jaw-bone and then its velocity and spin caused complete disintegration of the bone.'. The bullet had also disintegrated leaving fragments in the wound.
When the car was examined they found several areas that had been damaged including the windscreen and its frame just as in Kennedy's case, "each of which looked as if it had been caused by a separate bullet". He goes on to say that when the car was examined it was thought that a number of bullets must have been fired and that from the holes in the windscreen alone it seemed that two or three bullets had been fired. There was solid evidence and good witness testimony that only one bullet had been fired. Upon entry the bullet's fragments had dispersed in all directions taking the man's flesh with them and causing various damage to the car. The largest fragment - the bullet's tip, had, instead of flying forward in the direction of fire, flown off almost at right angles to the man's jaw and then ricocheted off the windscreen's frame and landed on the car's back seat. Sheldon also goes on to relate several more incidents with similar explosive wounds.
What he couldn't have known at the time was that he was describing not just a British .303 round but a MK VII .303 round. The difference is that these have the top third composed of aluminium, the other two thirds lead, underneath their usual copper jacket. He had noticed the bullet's aluminium tips earlier in the book and had thought them very useful for telling apart those who had been killed by the British from those killed by rioting Egyptians (much to his superiors distaste).
What these bullets do, and I'm not suggesting an aluminium bullet was used in the Kennedy assassination, is to offset the balance of the bullet creating just such a wobble as he first described. When such a bullet hits a substantial amount of flesh or a hard object such as bone, it shatters, releasing all of its energy at once causing the kind of explosive injuries described. Regular high-velocity bullets often pass right through a person taking their energy with them and leaving the opponent still able.