THE BUSHMASTER .223-caliber semiautomatic rifle that Adam Lanza carried into Sandy Hook Elementary School on Friday is a frightful killing machine. Just take a look at the manufacturer’s catalogue for this best-selling firearm. “At home on the range and on patrol,” it boasts. “Take on the predators with devastating performance.”

The Bushmaster is known for a design that can be configured for hunting, police work, range shooting and more. But the last place on earth this weapon should have been was in an elementary school. On Friday, Mr. Lanza wielded a Bushmaster to kill 20 children ages 6 and 7 and six adults in a rampage at the school that lasted some 10 minutes. He carried high-capacity magazines allowing him to fire off 30 bullets before he had to reload. A coroner said most of the victims were hit with at least three bullets — and some with up to 11 — that exploded with devastating lethality, tearing them apart from inside. The Bushmaster was a juggernaut of death.

Obviously, something snapped in this young man that points to evil beyond comprehension. Yet the tragedy leads to an inescapable conclusion. There is no defensible reason for civilians to own a Bushmaster or other semiautomatic rifles, known more broadly as assault weapons. This is essentially a military weapon, a version of the M-16, capable in some cases of shooting bullets at more than 2,000 feet per second. It does not belong in private hands, any more than M-1 Abrams tanks belong on Rockville Pike or mortars in the backyard.

Gun-owner advocates who point out that violent crime has fallen in recent years, in the absence of new gun controls, are correct. The link between guns and violence is not always clear-cut. But the link between a certain kind of mass murder and a particular killing technology is clearer. It makes sense to reimpose strict limits on these assault weapons, including a ban on high-capacity magazines with more than 10 bullets each. These magazines have figured in several of the most heinous mass shootings in recent years, including the one that severely wounded Rep. Gabrielle Giffords (D-Ariz.) in 2011. The Bushmaster is also a source of grief for Mexico, where it has become a favorite weapon of warring drug cartels.

In his remarks in Newtown on Sunday, President Obama noted that this was the fourth time in his presidency he has consoled a community grief-stricken by a mass shooting. He made clear he no longer wants to be simply the consoler in chief.

“We can’t tolerate this anymore,” he said. “No single law — no set of laws can eliminate evil from the world, or prevent every senseless act of violence in our society. But that can’t be an excuse for inaction. Surely, we can do better than this.”

Yes, we can. We can start by locking up these killing machines. Mr. Obama endorsed an assault weapons ban when running for president in 2008, but in office he has been cautious. The loss of life in Connecticut demands a move from caution to action.