...aimed semiautomatic fire from a competent shooter can be far more dangerous than automatic fire, which is harder to control and is often inaccurat… - C. J. Chivers

The main functional difference between the military’s M16 and M4 rifles and a civilian AR-15 is the “burst” mode on many military models, which allow three rounds to be fired with one trigger pull. Some military versions of the rifles have a full automatic feature, which fires until the trigger is released or a magazine is empty of ammunition.
But in actual American combat these technical differences are less significant than they seem. For decades the American military has trained its conventional troops to fire their M4s and M16s in the semiautomatic mode — one bullet per trigger pull — instead of on “burst” or automatic in almost all shooting situations. The weapons are more accurate this way, and thus more lethal.
The National Rifle Association and other pro-gun groups highlight the fully automatic feature in military M4s and M16s. But the American military, after a long experience with fully automatic M16s reaching back to Vietnam, decided by the 1980s to issue M16s, and later M4s, to most conventional troops without the fully automatic function, and to train them to fire in a more controlled fashion.
What all of this means is that the Parkland gunman, in practical terms, had the same rifle firepower as an American grunt using a standard infantry rifle in the standard way...
A New York Times analysis of a video from a Florida classroom estimates that during his crime the gunman fired his AR-15 as quickly as one-and-a-half rounds per second. The military trains soldiers to fire at a sustained rate of 12 to 15 rounds per minute, or a round every four or five seconds.

Many factors determine the severity of a wound, including a bullet’s mass, velocity and composition, and where it strikes. The AR-15, like the M4 and M16 rifles issued to American soldiers, shoots lightweight, high-speed bullets that can cause grievous bone and soft tissue wounds, in part by turning sideways, or “yawing,” when they hit a person. Surgeons say the weapons produce the same sort of horrific injuries seen on battlefields.
Civilian owners of military-style weapons can also buy soft-nosed or hollow-point ammunition, often used for hunting, that lacks a full metal jacket and can expand and fragment on impact. Such bullets, which can cause wider wound channels, are proscribed in most military use.

Classified reports from Vietnam were giving the AR-15 high marks and providing a surprise. Reports from the field claimed that when a bullet fired from the AR-15 struck a man, it inflicted devastating injuries.
The causes were apparently twofold. First, the metal jacket of early AR-15 bullets tended to shatter on impact, sending fragmentation slicing through victims. (In the army, this was variously seen as attractive and worrisome. In classified correspondence, some officers were thrilled by the perceived wounding characteristics, which one prominent army doctor described as "explosive effects." Others wondered whether the .223 round might be illegal under international convention.) Second, the bullets often turned sideways inside a victim, a phenomenon known as yaw. In one respect, the effects of yaw somewhat resemble what could be seen on the surface of a lake when a speedboat turned sharply. In this case, the energy delivery manifested itself as a shock wave within a human body, which could create stretching or rupturing injury to tissue not directly in a bullet's path. By turning, the bullet also crushed and cut more tissue as it passed through a victim, creating a larger wound channel.