In the ten years since the federal assault weapons ban expired, Sen. Dianne Feinstein, D-Calif., has kept trying to renew the law, which she authored. In a press release this month honoring the 20th anniversary of the ban, she wrote, "The evidence is clear: the ban worked."

But gun violence experts say the exact opposite. "There is no compelling evidence that it saved lives," Duke University public policy experts Philip Cook and Kristin Goss wrote in their book "The Gun Debate: What Everyone Needs to Know."

A definitive study of the 1994 law – which prohibited the manufacture and sale of semiautomatic guns with "military-style features" such pistol grips or bayonet mounts as well as magazines holding more than ten rounds of ammunition – found no evidence that it had reduced overall gun crime or made shootings less lethal. "We cannot clearly credit the ban with any of the nation's recent drop in gun violence," the Department of Justice-funded study concluded in 2004. "Should it be renewed, the ban's effects on gun violence are likely to be small at best and perhaps too small for reliable measurement."

As we recently reported, key gun control groups say they are no longer making an assault weapons ban a priority because they think focusing on other policies, including universal background checks, are a more effective way to save lives. The Center for American Progress released a report earlier this month suggesting ways to regulate assault weapons without banning them.

Feinstein introduced an updated version of the assault weapons ban last year, in the wake of the mass shooting at Sandy Hook Elementary School, in which the shooter used a type of rifle that had been targeted by the ban. She told her Senate colleagues to "show some guts" when they voted on it in April. The measure failed, 40 to 60. The push to improve background checks also failed, but attracted more support. 

The key statistic that Feinstein cited in her recent press release — that the ban "was responsible for a 6.7 percent decrease in total gun murders, holding all other factors equal"— was rejected by researchers a decade ago.

Feinstein attributed the statistic to an initial Department of Justice-funded study of the first few years of the ban, published in 1997.

But one of the authors of that study, Dr. Christopher Koper, a criminologist from George Mason University, told ProPublica that number was just a "tentative conclusion."  Koper was also the principal investigator on the 2004 study that, as he put it, "kind of overruled, based on new evidence, what the preliminary report had been in 1997."

Feinstein's spokesman, Tom Mentzer, contested the idea that the 2004 study invalidated the 1997 statistic that Feinstein has continued to cite. But Koper said he and the other researchers in 2004 had not re-done the specific analysis that resulted in the 6.7 percent estimate because the calculation had been based on an assumption that turned out to be false. In the 1997 study, Koper said, he and the other researchers had assumed that the ban had successfully decreased the use of large-capacity magazines. What they later found was that despite the ban, the use of large-capacity magazines in crime had actually stayed steady or risen.

"The weight of evidence that was gathered and analyzed across the two reports suggested that initial drop in the gun murder rate must have been due to other factors besides the assault weapons ban," Koper said.

Cook, the Duke public policy expert, told ProPublica that the "weak results" of the 1994 ban "should not be interpreted to mean that in general bans don't work."

He said Feinstein's updated version of the ban, which she proposed in 2013 and is more restrictive, might be more effective. An American assault weapons ban might also have an impact on drug and gang-related violence in Mexico, he said.

"Around 30,000 Americans are killed with guns each year; one-third of those are murders," Feinstein said in a statement to ProPublica. "Obviously there's no single solution, which is why I support a wide range of policy proposals to bring sense to our firearms laws.  I continue to believe that drying up the supply of military-style assault weapons is an important piece of the puzzle—and the data back this up." (See Feinstein's full statement below.)

Gun rights groups have long criticized the ban, and Feinstein's defense of it.

"Gun rights organizations, Second Amendment people, always take Dianne Feinstein with the whole shaker full of salt," said Dave Workman, the communications director for the Citizens Committee for the Right to Keep and Bear Arms.  "She's been a perennial gun-banner." 

"One would think the lesson learned from banning alcohol, marijuana, and many other drugs and items [is that] it never works for anyone intent on obtaining any of these items," Jerry Henry, the executive director of GeorgiaCarry.org, told ProPublica. "All it does is put it in the background and helps establish a flourishing black market."  

 The National Rifle Association did not respond to a request for comment.

Full Feinstein statement:

"Around 30,000 Americans are killed with guns each year; one-third of those are murders. Obviously there's no single solution, which is why I support a wide range of policy proposals to bring sense to our firearms laws. We need to expand background checks, strengthen gun trafficking laws and make sure domestic abusers, the seriously mentally ill and other dangerous people cannot access guns.

"I continue to believe that drying up the supply of military-style assault weapons is an important piece of the puzzle—and the data back this up. These weapons were designed for the military and have one purpose: to kill as many people as possible, as quickly as possible. They are the weapon of choice for grievance killers, gang members and juveniles, and they shouldn't be on the streets.

"A 2004 Justice Department study found clear evidence that the ban on manufacture and transfer of assault weapons reduced their use in crimes. The percentage of assault weapons traced as part of criminal investigations dropped 70 percent between 1993 and 2002, and many police departments reported increases in the use of assault weapons after the ban expired. In less than a decade, the ban was already drying up supply. The study suggested the law would have been even more effective if it had banned weapons already in circulation and if it had continued past its 10-year duration. Unfortunately those limits were part of the compromise that had to be struck to pass the ban into law.

"Let me be clear: Assault weapons allow criminals to fire more shots, wound and kill more individuals and inflict greater damage. The research supports that. A ban on assault weapons was never meant to stop all gun crimes, it was meant to help stop the most deadly mass shootings. That's why it needs to be a part of the discussion, or rampages like Sandy Hook will continue to happen."