The National Firearms Act includes much of what the N.R.A. fights against, but the lobbying group hasn’t directly challenged it. That may be because the firearms database, which includes weapons owned by both private citizens and law enforcement agencies, accounts for only a small percentage of the 300 million firearms estimated to be in private hands. Perhaps it should fight against it, though, because the 1934 law makes a good case against the N.R.A. The more than four million weapons inventoried — including machine guns, missiles, hand grenades and mortars — are the best example we have of gun control that works.

The N.F.A. was designed to control what today’s Justice Department calls “dangerous weapons that empower a single individual to take many lives in a single incident.” Millions of firearms now in private hands, including assault rifles designed for use by the military, are just as lethal as weapons covered by the N.F.A. They should be brought under the act.

Jeff Folloder, the executive director of the N.F.A. Trade and Collectors Association, says his members have learned to live with gun registration and lose no sleep worrying about confiscation. “There are still an enormous number of people who think if they register and purchase an N.F.A. weapon, they’re giving A.T.F. permission to come knock on their door at any time, and that’s just not true,” Mr. Folloder told me. “You’re not giving up any rights.”

Mr. Folloder, a manufacturer and collector of machine guns and silencers, is no fan of the firearms act. He’d like to make machine guns more readily available, and lobbied successfully to repeal a requirement that N.F.A. owners be preapproved by a local law enforcement officer. But Mr. Folloder and the A.T.F. agree that registrants are rarely implicated in crimes. People with registered weapons who can pay $40,000 for a machine gun “bend over backwards to obey the law,” he says. “If you’re going to spend that much money and put that much effort into obtaining one of these, you’re not going to be holding up the liquor store.”

N.F.A.-classified weapons do show up at crime scenes. But nearly all of them were unregistered, so the simple act of possession was a crime. According to A.T.F. analysis, among N.F.A. weapon owners there were only 12 felony convictions between 2006 and 2014, and those crimes did not involve an N.F.A. weapon. If that conviction rate were applied to the owners of the other privately owned firearms in the United States, gun crime would virtually disappear.