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Everything you need to know about the assault weapons ban, in one post

When Adam Lanza shot 26 people in Sandy Hook Elementary School on Friday, police say he largely relied on a Bushmaster AR-15 "assault-type weapon," a semiautomatic rifle that could rapidly fire multiple high-velocity rounds. He was also equipped with magazines that held 30 bullets each.

Advocates of stricter gun control argue that it shouldn't be so easy to have access to weapons that powerful — or to magazines capable of holding so many bullets. And so, on Sunday, Sen. Dianne Feinstein (D-Calif.) said she would introduce new legislation to ban assault weapons at the start of the next Congress. President Obama has also said that he'd support a federal ban.

But how much would this legislation do? Back in 1994, Congress passed a federal assault-weapons ban that lasted 10 years. Experts who have studied the law tend to agree that it was rife with loopholes and generally ineffective at curbing gun violence — though it might well have reduced mass shootings.

Here's a look at what the 1994 law actually did, where it failed, and whether it could be reworked to significantly reduce gun violence.

What counts as an "assault weapon"? The trouble all starts here. There's 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 semiautomatic weapons that reload automatically but fire only once each time the trigger is depressed. Semiautomatic pistols and rifles come in all shapes and sizes and are extremely common in the United States.

Congress didn't want to ban all semiautomatic weapons — that would ban most guns, period. So, in crafting the 1994 ban, lawmakers mainly focused on 18 specific firearms, as well as certain military-type features on guns. Complicated flow charts laid it all out. Certain models of AR-15s and AK-47s were banned. Any semiautomatic rifle with a pistol grip and a bayonet mount was an "assault weapon." But a semiautomatic rifle with just a pistol grip might be okay. It was complicated. And its complexity made it easy to evade.

What did the 1994 ban actually do? For the 10 years that the ban was in effect, it was illegal to manufacture the assault weapons described above for use by private citizens. The law also set a limit on high-capacity magazines — these could now carry no more than 10 bullets.

There was, however, an important exception. Any assault weapon or magazine that was manufactured before the law went into effect in 1994 was perfectly legal to own or resell. That was a huge exception: At the time, there were roughly 1.5 million assault weapons and more than 24 million high-capacity magazines in private hands.

Did the 1994 law have loopholes? Yes, lots. Even after the ban took effect, it was not difficult for someone to get their hands on an assault weapon or high-capacity magazine.

A 2004 University of Pennsylvania study commissioned by the National Institute of Justice explained why. For starters, only 18 firearm models were explicitly banned. But it was easy for gun manufacturers to modify weapons slightly so that they didn't fall under the ban. One example: the Colt AR-15 that James Holmes used to shoot up a movie theater in Aurora, Colo., last summer would have been outlawed. Yet it would have been perfectly legal for Holmes to have purchased a very similar Colt Match Target rifle, which didn't fall under the ban.

Meanwhile, here were already more than 24 million large-capacity magazines in existence before the federal ban took effect in 1994. Indeed, as soon as Congress began working on the law, manufacturers boosted production of weapons and magazines in anticipation of higher prices. Dangerous weapons were still plentiful.

Did the law have an effect on crime or gun violence? While gun violence did fall in the 1990s, this was likely due to other factors. Here's the UPenn study again: "We cannot clearly credit the ban with any of the nation’s recent drop in gun violence. And, indeed, there has been no discernible reduction in the lethality and injuriousness of gun violence."

One reason is that assault weapons were never a huge factor in gun violence to begin with. They were  used in only 2 percent to 8 percent of gun crimes. Large-capacity magazines were more important — used in as many as a quarter of gun crimes. But, again, the 1994 law left more than 24 million magazines untouched, so the impact was blunted.

Did the law have an effect on mass shootings? That's possible, though not certain. As this chart from Princeton's Sam Wang shows, the number of people killed in mass shootings did go down in the years the ban was in effect (save for a surge in 1999, a year that included Columbine):

Because mass shootings are relatively rare, it's difficult to tell whether this was just a random blip or caused by the ban. Still, the number of mass shootings per year has doubled since the ban expired. That's suggestive, at least.

Why did the ban lift in 2004? The original assault weapons law was written so that it would expire after ten years. When 2004 came around, some Democrats tried to renew it, but there wasn't much interest in Congress. A few states, including New York, Massachusetts and New Jersey, still have their own versions of the law on the books.

Could the ban have worked if it had lasted longer? In his 2004 assessment of the law, the University of Pennsylvania's Christopher Koper argued that any federal ban would likely take many years to have an effect, thanks to all the exemptions. If Congress actually managed to reduce the number of high-capacity magazines, that might slightly reduce the number of gunshot victims. But, Koper added, the research here was fairly thin and any predictions were "tenuous."

Would it be possible to tighten the law? In theory, yes. Back in 1996, Australia imposed a much stricter version of the assault weapons ban after a mass shooting. The Australian version avoided many of the loopholes in the U.S. law: Not only did the country ban all types of semiautomatic rifles and shotguns, but it also spent $500 million buying up nearly 600,000 existing guns from private owners.

As Wonkblog's Sarah Kliff pointed out, Australia's law appears to have curbed gun violence. Researchers in the British Medical Journal write that the ban was “followed by more than a decade free of fatal mass shootings, and accelerated declines in firearm deaths, particularly suicides.”

Still, an Australia-style ban would face much more difficult hurdles in this country. For starters, there are more than 200 million guns in circulation in the United States, making a buyback much more costly. And a full ban would likely face heavier resistance here, both from the courts and the public. Even Feinstein has promised that her new version of the assault weapons ban would still "exempt over 900 specific weapons." Gun-control advocates aren't quite ready to propose overly sweeping measures.

Related: Twelve facts about guns and mass shootings in the United States.