It was easily adapted for civilian use, with one major difference: Military versions can fire fully automatically — or in bursts of several shots — by depressing the trigger once. The civilian semiautomatic version requires a pull of the trigger for each shot.

But other features that make the AR-15 so deadly on the battlefield remain. It is light, easy to hold and easy to fire, with a limited recoil. Bullets fly out of the muzzle more than twice as fast as most handgun rounds.

Equally important for a gunman looking to do a lot of damage in a hurry: AR-15-style weapons are fed with box magazines that can be swapped out quickly. The standard magazine holds 30 rounds. Equipped in this way, a gunman can fire more than a hundred rounds in minutes.

The man accused of the killings in Parkland had “countless magazines” for his AR-15, the local sheriff said.

And there is still one more reason the weapons are so popular in states like Florida: They are very easy to buy — and for a 19-year-old like Nikolas Cruz, the shooting suspect, far easier to obtain than a handgun.