For example, Rep. Carolyn McCarthy, D‐​New York, wants to outlaw magazines with more than 10 rounds — even those already in circulation. She hasn’t explained how a ban on previously sold magazines would deter anyone but law‐​abiding citizens.

Still, the Supreme Court has suggested that sensible gun regulations may be constitutionally permissible. Sensible is not, however, what we have in Washington, Chicago, New York and other cities, where you can probably get a pizza delivery before a response from a 911 call. Police cannot be everywhere.

Selected proposals may nonetheless be constructive, with three important qualifications. First, government has the burden to show that a regulation will not unduly impede the use of firearms for self‐​defense. Second, ostensibly modest steps down a slippery slope must not compromise core Second Amendment rights. Third, a regulation must be effective in promoting public safety, when weighed against reliable evidence that past restrictions have not lessened the incidence of gun‐​related crimes.

Recall that Washington banned handguns for 33 years; during some of those years the city was known as the nation’s murder capital. Killers not deterred by laws against murder were not deterred by laws against owning guns. Moreover, anti‐​gun regulations did not address the deep‐​rooted causes of violent crime — illegitimacy, drugs, alcohol abuse and dysfunctional schools — much less mental instability.

In 2004, the National Academy of Sciences reviewed 253 journal articles, 99 books and 43 government publications evaluating 80 gun‐​control measures. Researchers could not identify a single regulation that reduced violent crime, suicide or accidents. A year earlier, the Centers for Disease Control reported on ammunition bans, restrictions on acquisition, waiting periods, registration, licensing, child access prevention and zero tolerance laws. CDC’s conclusion: There was no conclusive evidence that the laws reduced gun violence.

So much for the quasi‐​religious faith that more controls mean fewer murders. There are about 500,000 gun‐​related crimes annually in the United States. Further, Americans own roughly 250 million guns. Assuming a different gun is used in each of the 500,000 crimes, only 0.2% of guns are involved in crime each year. A ban on firearms would be 99.8% over‐​inclusive.

We should also resist seemingly measured gun controls such as raising the age limit from 18 to 21, requiring background checks at gun shows, and reinstating the assault weapons ban.

Eighteen‐​year‐​olds are allowed to vote, go to war, get married and divorced, and have an abortion. Maybe that cutoff is too low; but an individual sufficiently mature to engage in those activities is responsible enough to own a gun for self‐​defense.

Gun shows did not play a role in Jared Loughner’s rampage. He apparently acquired his firearm at a retail store through a licensed dealer. In fact, merely 2% of guns used by criminals are purchased at gun shows. That includes straw purchases, which are already illegal, and purchases through dealers, which are subject to background checks. Nearly all rejected buyers turn out to be false positives. Others, bent on committing crimes, just shop elsewhere, on the black market if necessary.

Expiration of the assault weapons ban in 2004 did not — contrary to popular belief — legalize automatic firearms. Those weapons have been banned since 1934 for all practical purposes. The ban covered semi‐​automatic weapons, which are used by tens of millions of Americans for hunting, self‐​defense, target shooting, and even Olympic competition. Take it from The New York Times, written a few months after the ban expired: “Despite dire predictions that the streets would be awash in military‐​style guns, expiration of the assault weapons ban has not set off a sustained surge in sales (or) caused any noticeable increase in gun crime.”

The U.S. Constitution imposes no obstacle to more thorough screening of gun applicants for mental impairment. Nor does the Constitution likely preclude reasonable background checks, or even tighter constraints on high‐​capacity magazines. Those proposals, which may reflect the common‐​sense views of many Americans, must be tempered by this reality: Experience indicates that gun restrictions have minimal effect on access to weapons by criminals and deranged people.