A sign advertising a “Home Defense Special” at the Shooters Shop in West Allis, Wisconsin. Credit: CORBIS

Loopholes in existing state and federal laws continue to allow many types of deadly weapons to be sold to both criminals and people with mental health problems. But fixing these loopholes may bump up against constitutional and civil liberties, according to some legal experts.

Still, advocates say they want President Obama and Congress to take swift steps rather than spending more time studying the issue of gun control.

At a Capitol Hill press conference on Tuesday, family members of victims of mass shootings gathered in the shadow of the House of Representatives to press their case with lawmakers. Dan Gross, president of the Brady Center to Prevent Gun Violence said his biggest priority is closing the so-called “gun show loophole” that allows private dealers to sell guns to individuals without a criminal background check.

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Gross said that 40 percent of guns sales are done without such a federally-mandated check, often at gun shows or in parking lots. Gun sales at retail sporting good stores and licensed gun shops do have perform such a check for domestic violence convictions or court-mandated restrictions on gun ownership for mental health reasons.

“It’s a gaping hole in our nation’s background check system,” Gross told Discovery News.

Even licensed gun dealers who sell away from their principal place of business are exempt from running background checks, according to a study by the Congressional Research Service.

In addition to limiting gun sales, the other issue is limiting the kinds of people who can obtain weapons. Under federal law, people who have been ruled mentally incompetent or involuntarily committed have their names put into a federal database that is used during gun sales.

But a study earlier this year by Mayors Against Gun Violence found that the federal database lacked millions of mental health records that would make it an effective deterrent.

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In fact, 21 states had submitted less than 100 names each to the criminal background list. Some state officials have blamed delays in switching from paper to computer record-keeping, or conflicts with existing state statutes that protect the privacy of mental health patients.

That doesn’t wash with Omar Samaha.

“There’s so much more that we can do that we’re not doing,” said Samaha, a former Virginia Tech student whose sister Reema was among the 32 people shot and killed during the April 16, 2007, rampage by Seung-Hui Cho. “We’ve haven’t done enough as a country to prevent dangerous individuals from buying guns.”

Samaha noted that Cho had a history of mental illness but his name wasn’t added to the criminal background list because of failures in Virginia’s social welfare system.

One expert says that it may be easier to regulate a person’s ability to purchase rather than the production of the weapons themselves.

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The assault weapons ban that lasted from 1994 to 2004 didn’t work very well because it focused on the look of the gun rather its capabilities or who could own it, according to Robert J. Cottrol, professor of law at George Washington University.

“That was really a ban on cosmetic features and not really meaningful legislation,” Cottrol said.

The ban prohibited semi-automatic weapons that had a certain kind of bayonet lug, pistol grip or flash suppressors. “None of which have any bearing on how dangerous the rifle might be,” he said. “What we really need to regulate is who gets guns.”

Trying to devise legislation and policy to regulate the intersection of mental health and guns could be a challenge. Not all mental health conditions lead to violence, and not all people suffering from mental health problems are being treated by someone who could determine whether or not they are a danger to themselves or others.

The tipping point in any legislation likely to be taken up by Congress in the weeks after the Newtown, Conn., elementary school shooting is the balance between individual privacy and public safety.

“We need more access to mental health records and some kind of procedure where authorities can ask to have an individual placed on firearms risk list for mental health reasons,” Cottrol said.

Sen. Diane Feinstein, D-Calif, and author of the original assault weapons ban, has said this week she plans to reintroduce similar legislation. Some pro-gun Democrats, including Sen. Joe Manchin of West Virginia and Mark Warner of Virginia, have said they are switching their positions and will support new federal rules on guns.

Any new legislation is likely to face opposition from gun rights’ groups that spent $3.7 million during the 2012 election season, compared to $180,000 from gun-control advocates, according to the Center for Responsive Politics.