Why is the 5.56mm round so deadly in school shootings?
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Flight ballistics: The bullet exits the muzzle of the AR-15 (or M4) unstable. The bullet is not merely spinning around its long axis (the front to rear line). It also "yaws" circularly, up to 4 degrees off center line (a form of gyroscopic precession). At about 100 meters, however, the yaw disappears and the round flies very stable out to about 400 meters, when it starts yawing again.
Victims of school shootings are all shot at ranges of much less than 100 meters. So the bullet strikes them while still yawing. That directly affects what happens to them,
Terminal, or wound, ballistics: ...
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1. The 5.56 round exits the rifle at just more than 1,000 meters per second, about three times the speed of sound. When the 5.56 bullet hits a human body, it immediately begins to decelerate. This bullet's length to width ratio is high. The nose of the bullet begins to decelerate but the rear is still traveling supersonic. The rear is going faster than the nose. This causes the rear to overtake the nose, meaning that the bullet begins to tumble end over end. This tumbling in turn causes the bullet to fragment not quite completely and the fragments travel through flesh, bounce off bone into new directions and sever nervous system connections. This tumbling is greatly enhanced if the bullet is yawing at impact, as every bullet fired during a school shooting is. That is part one of what causes instant incapacitation.
2. Many ballisticians also say that the supersonic shock wave, shaped like a cone pointed in the direction the bullet was flying, enters the entrance wound and expands it rapidly for a distance into the body (how much depends on the location of the entrance wound and the angle). This causes excess bleeding over what the bullet would cause alone. This effect is called "hydrostatic shock," but not all armaments ballisticians agree that it is significantly damaging. In fact, while sound travels four times faster through human tissue than through the air, sonic waves have been proven not to damage the body.
Cavitation: Any high-velocity bullet (and almost all non-HV bullets, too) not only creates a cavity in the body corresponding to the width of the bullet, they also create large-diameter cavities from the sudden displacement of tissue caused by the speed of the bullet. This is called "cavitation." A radiologist who helped treat victims at Parkland described it this way:
The bullet from an AR-15 passes through the body like a cigarette boat traveling at maximum speed through a tiny canal. The tissue next to the bullet is elastic—moving away from the bullet like waves of water displaced by the boat—and then returns and settles back. This process is called cavitation; it leaves the displaced tissue damaged or killed. The high-velocity bullet causes a swath of tissue damage that extends several inches from its path. It does not have to actually hit an artery to damage it and cause catastrophic bleeding. Exit wounds can be the size of an orange.
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more (pretty detailed, and gruesome):
https://senseofevents.blogspot.com/2018/02/why-ar-15-is-very-deadly-military-weapon.html
I.e. every one of those white dots is a bullet fragment.
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