Mass public killings are too common in America, but the murder of dozens of concert-goers in Las Vegas Sunday night had a unique factor. The shooter used a fully-automatic rifle to mow down the victims, of whom there were more than 500.

Reporting and commentary on guns, especially in the harrowing aftermath of mass shootings, is regularly muddled, and the press often conflates automatic weapons and semi-auto weapons. Semi-automatic weapons fire one round per trigger pull, while an automatic weapon can spray many rounds per second as long as the shooter squeezes the trigger.