We've been here before. After Patrick Purdy sprayed a Stockton elementary school with a semi-automatic rifle in 1989, California legislators passed a law banning assault weapons. A few years -- and massacres -- later, in 1994, President Bill Clinton signed into law a 10-year nationwide ban sponsored by Calif. Sen. Diane Feinstein.

U.S. Gun Sales Reach Record Levels In 2012 — A California ban on assault weapons continues, but a federal ban has expired. (Scott Olson/Getty Images)

Now assault weapons have exploded back onto the national agenda. Feinstein has been trying to renew the law since it expired in 2004, and last week’s mass murder in Newtown, Conn., prompted President Barack Obama to endorse that effort.

At the same time, California Treasurer, Bill Lockyer has proposed that some state pension plans should divest from companies making guns banned by California. And an inquiry from a California state teachers fund led a private equity firm to put the company that made the gun used in the Connecticut killings up for sale. Other proposals are surfacing as well. State Sen. Kevin De Leon (D-Los Angeles) has proposed requiring permits for the purchase of ammunition. San Francisco Mayor Ed Lee on Thursday called for a ban on hollow-point bullets.

As with any discussion on gun policy, passions run high -- and so does confusion. Here are some of the more common misconceptions about assault weapons:

1) Assault weapons are more powerful than any hunting weapon. Before 1989, the term "assault weapon" had virtually no meaning. Although the term "assault rifle" was sometimes used to refer to a type of light semiautomatic military rifle beginning in World War II, it did not have a precise definition. Both Patrick Purdy and the Newtown, Conn. shooter, Adam Lanza, reportedly used copies of rifles designed for military use -- in Purdy's case a Norinco Type 56 Assault Rife (a Chinese copy of the Russian AK-47), and in Lanza's case a Bushmaster AR15 (the civilian version of the U.S. military's M-16).

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But these guns are no more powerful than many semiautomatic rifles legally used for hunting in California and throughout the United States. They don't shoot farther, faster or with more power. In order to create an "assault weapon" ban, legislators had to list specific models of guns or characteristics such as pistol grips on rifles, flash hiders, folding rifle stocks and threaded barrels for attaching silencers.

2) Assault weapons kill more people than other guns. Assault weapons as defined by either the California or U.S. legislation are not used in most murders. A University of Pennsylvania 2004 study found that guns covered by the U.S. ban made up less than 8 percent of the guns used in crimes before the law passed. Even when you narrow the type of crime down to mass murders or police killings, assault weapons were implicated in less than 15 percent.

So what category of guns are implicated in the most deaths? "Handguns are used in three-fourths of American fatal shootings," says Franklin Zimring, a University of California law professor. And they make up much less than three-quarters of the guns in the hands of U.S. civilians, so they are disproportionately dangerous, he said.

The University of Pennsylvania study found that assault-weapon use declined in the four cities it examined, Baltimore, Milwaukee, Louisville, and Anchorage, by about 17 percent during the period the law was in place. But it concluded that other guns that could be loaded with large magazines took the place of the banned weapons. (Magazines for semiautomatic guns, also known as clips, are typically detachable, so the same gun could hold a large magazine or a small one.) While the federal law banned magazines with a capacity of more than 10 bullets, it did nothing to remove the 25 large magazines already on the market -- about one in five civilian U.S. guns.

3) Studies show that assault weapons bans do/don't work. The University of Pennsylvania study concluded in 2004 that it was "premature" to reach conclusions about whether the assault weapons ban had worked. Volokh, who thinks gun control can't work, and Zimring, who thinks it can, agree on one thing: the research so far hasn't settled the question. Reports by the National Academies of Science and the Centers for Disease Control and Prevention reached the same conclusions: not enough data.

4) Assault weapons are machine guns. Machine guns are already illegal for civilian use in the United States. "This is not about machine guns," says University of California, Los Angeles law professor Eugene Volokh. "It's about semiautomatic guns that are not materially different from other kinds of semiautomatic weapons."

A 1934 law required special permits for machine guns, and a 1986 law further restricted them. The term semiautomatic means that the gun automatically puts a new bullet in its chamber each time the trigger is pulled, until the magazine that holds its ammunition is empty. But to fire another bullet, the shooter has to release the trigger, then squeeze it again. By contrast, a fully automatic gun keeps firing as long as the trigger is held down.

That leaves policy makers to rely on their common sense -- or prejudices -- in drawing up new laws.

For more about assault weapons, read the Washington Post's blog post, "Everything You Need to Know about Banning Assault Weapons in One Post."

Patrick PurdyPatrick Purdy

4 Myths About Assault Weapons