Newtown. San Bernardino. Las Vegas. Sutherland Springs. And now, Parkland. Five of the six deadliest mass shootings of the past six years in the Unit… - Richard A. Oppel, Jr.

The N.R.A. calls the AR-15 the most popular rifle in America. The carnage in Florida on Wednesday that left at least 17 dead seemed to confirm that the rifle and its variants have also become the weapons of choice for mass killers.

Among the six deadliest mass shootings in the last six years, the only one not carried out with an AR-15 style rifle was the Orlando nightclub attack two years ago that left 49 dead. The gunman, Omar Mateen, had a Sig Sauer MCX semiautomatic rifle, which shares features with the AR-15, though it functions via a different semiautomatic design — and is just as deadly. All of these military-style semiautomatic weapons have something else in common. They have been heavily marketed as home-defense and marksmanship weapons, and their sales have been a major driver of profits for gun manufacturers over the past two decades.

A vast majority of guns used in 19 recent mass shootings were bought legally and with a federal background check. At least nine gunmen had criminal histories or documented mental health problems that did not prevent them from obtaining their weapons.