The debate over gun control in the United States has waxed and waned over the years. Each time there is a mass killing by gunmen in civilian settings, there is outrage—for what seems like 10 seconds. In particular, the killing of 20 schoolchildren and six educators in Newtown in December 2012 fueled a national discussion over gun laws, with calls by the Obama administration to limit the availability of military-style weapons.

But despite extensive public support, legislation to ban semi-automatic assault weapons and expand background checks was defeated in the Senate in 2013. Deadly mass shootings in 2015, including the killing of nine people at a church in Charleston, South Carolina, and 14 at a community center in San Bernardino, California, helped to rekindle the debate. But little has changed.

The Gun Control Act of 1968 prohibited the sale of firearms to several categories of individuals, including persons under 18 years of age, those with criminal records, the mentally disabled, unlawful aliens, dishonorably discharged military personnel and others. In 1993, the law was amended by the Brady Handgun Violence Prevention Act, which mandated background checks for all unlicensed persons purchasing a firearm from a federally licensed dealer. But by 2016, there were no federal laws banning semi-automatic assault weapons, military-style .50 caliber rifles, handguns, or large-capacity ammunition magazines. The federal prohibition on assault weapons and high-capacity magazines between 1994 and 2004 had expired.

Not that that prohibition was so great. First, there is no technical definition of an “assault weapon.” There are fully automatic weapons, which fire continuously when the trigger is held down. Those have been strictly regulated since 1934. Then there are semi-automatic weapons that reload automatically but fire only once each time the trigger is depressed. Semi-automatic pistols and rifles come in all shapes and sizes and are extremely common in the United States. Because Congress didn’t want to ban all semi-automatic weapons, lawmakers mainly focused on 18 specific firearms, as well as certain military-type features on guns. Complex flow charts laid it all out but it was extremely complicated, thereby making it easy to evade.

Additionally, as both gun control advocates and gun rights advocates noted, at least some of the features outlined in the federal Assault Weapon Ban of 1994 were merely cosmetic. In May 2012, the Law Center to Prevent Gun Violence said, “the inclusion in the list of features that were purely cosmetic in nature created a loophole that allowed manufacturers to successfully circumvent the law by making minor modifications to the weapons they already produced.” The term was repeated in several stories after both the 2012 Aurora and Sandy Hook shootings.

Moreover, there was an important exception. Any assault weapon or magazine that was manufactured before the law went into effect in 1994 was legal to own or resell, and at the time there were roughly 1.5 million assault weapons and more than 24 million high-capacity magazines in private hands. Further, as soon as Congress began working on the law, manufacturers increased production of weapons and magazines in anticipation of higher prices. But, as imperfect as it was, it was something.

In January 2016, President Barack Obama issued a package of executive actions designed to decrease gun violence, notably a measure to require dealers selling firearms at gun shows or online to obtain federal licenses and, in turn, conduct background checks of prospective buyers. Additionally, he proposed new funding to hire hundreds more federal law-enforcement agents, and budgeting $500 million to expand access to mental health care in light of the fact that suicides—many by individuals with undiagnosed mental illnesses—account for about 60 percent of gun deaths. The president acted under his own authority because Congress had failed to pass “common-sense gun safety reforms.” Congress continues to fail.

Mother Jones magazine tracks and maps every shooting spree in the last three-plus decades. It found that since 1982, there have been at least 91 mass murders carried out with firearms across the country, with the killings unfolding in 30 states from Massachusetts to Hawaii. And in most cases, the killers had obtained their weapons legally. The deadliest shooting came this week in Las Vegas, leaving 59 people dead and more than 500 injured. So once again, we ask: What’s it going to take for Congress to respond?