Correia’s transformation began when he asked his self-defense teacher how to guard against a knife attack. “The way we practiced didn’t seem right,” he told me. On YouTube, Correia had watched a few real-life stabbings caught on surveillance video, and “they didn’t look like what we were training against.” In the safety of the dojo, Correia and his classmates were practicing for an attacker who would extend his blade with one elegant thrust, like an Olympic fencer. “There was no energy, no resistance, no ill will,” he added. A real killer, the surveillance footage suggested, will hook you by the neck with one arm and plunge the knife into you repeatedly with the other, shredding your belly into strips of human bacon and chitterlings. “I asked him, ‘What do we do about this?’ ” The sensei, normally hard to stump, didn’t have an answer. “Right now,” he shrugged, “we die.”

Since then, Correia has watched approximately 13,000 more videos of deadly and near-deadly encounters, in an effort to bring reality to a field distorted by fantasy. As violence has become rarer, fewer people have had the misfortune of becoming personally acquainted with it. We harbor illusions about how muggings, gangland slayings, and bar fights go down, and about what we can do to intervene or protect ourselves. The new ubiquity of video surveillance could force gun nuts and gun haters alike to confront reality. Correia says he is “an evidence-based self-defense trainer”—a sabermetrician of violence who, having cataloged the events of each video, can tell you with nerdy accuracy that a third of attacks involve multiple assailants. Pepper spray works about 90 percent of the time. Twenty-three percent of the videos come from Brazil, so if you don’t want to be stabbed on camera, don’t go to Rio.

“Every situation is a snowflake,” he says. “But the same principles show up again and again. All I do is to teach people and give them a vocabulary for what to do.” In Correia’s most popular video, which is from Venezuela, an armed robber approaches his victim, an off-duty cop, in an ATM line. After dropping his wallet and necklace on the ground, the cop falls back, lets the mugger stoop to pick up the loot, and takes advantage of his distraction to draw a pistol and shoot the criminal four times. There are “some significant lessons here,” Correia says. He approves of the distraction. “This was incredibly wise … Give [the mugger] something else to think about. Don’t just stand there and fight him when he’s strong.” He commends the cop for “concealing his draw.” (The cop hid, somewhat ungallantly, behind a civilian for a few seconds to do so.) “This guy did a great job.”

Video: How to Win a Gun Fight

Correia’s narration is notable for its sanity and practicality, and (a rarity in the gun world) for not viewing all problems as solvable with more and larger guns. He used to carry more than one gun on his person, plus a spare mag in case he needed to reload. But in his study of violent encounters, he has seen zero emergency reloads and zero uses of a backup gun (or bug, in gun lingo), so he seldom carries extra mags anymore and has stopped carrying an extra gun altogether. He replaced them with a first-aid kit—which he has used twice, once to save a life—and pepper spray, which he has used twice to defend himself against stray dogs.