How will Newtown shootings change the US gun debate?

The Connecticut school shootings have horrified America. But what impact, if any, will they have on the nation's gun laws?

Even in a nation with a roll call of gun massacres so long and familiar - Columbine, Virginia Tech, Aurora, Jonesboro and many others - the killings at Sandy Hook Elementary have a singular capacity to shock.

Twenty children and six adults lie dead. A gunman who forced his way into the school took their lives and his own.

The tragedy has re-opened the debate over the nation's gun laws as long-term supporters of reform have issued calls for tighter controls.

After the shootings, an emotional President Barack Obama promised "meaningful action", adding: "As a country, we have been through this too many times."

According to a July 2012 study by the magazine Mother Jones of 62 mass murders carried out in the US since 1982, three quarters of the 139 firearms used by the killers were held legally. Of these, more than 60 were semi-automatic handguns and over 30 were assault weapons.

But in a country with an estimated 300m guns, where the right to bear arms is mentioned in the constitution, gun control advocates are wary of claiming that change is at hand.

Public support for stricter gun legislation has been on a downward trend in recent years, along with overall levels of violent crime.

This, after all, is a nation where the pro-gun National Rifle Association (NRA) has more than 4m members. According to the Small Arms Survey, there were 88.8 firearms for every 100 Americans in 2007.

Likewise, US legal and political trends have been moving away from supporters of greater controls, says James Jacobs, director of the Center for Research in Crime and Justice at New York University.

"All of the policy momentum in the last 20 years has been in the direction of gun owners' rights," he says.

The Supreme Court decided in 2008 that the US constitution's second amendment gives Americans the right to own guns for personal use, rather than just protecting the collective right of states to maintain militias.

The House of Representatives is currently controlled by the Republican party. The NRA endorses Republicans and Democrats, but has given significantly more campaign contributions to Republicans.

President George W Bush allowed a federal ban on assault weapons to expire in 2004.

Meanwhile, the backlash against Democratic politicians who passed gun control bills in 1993 and 1994 frightened centre-left candidates into steering clear of the issue, says Kristin Goss of Duke University.

"The Democrats have a belief that it's not a winning issue for them," she adds.

As a result, gun rights were barely an issue during the 2012 election, apart from when President Obama reaffirmed his support for an assault weapon ban in response to a question during one of the presidential debates.

Other countries have responded by tightening their gun laws following public outcries in the wake of mass shootings.

Access to firearms were restricted in the UK following the 1987 Hungerford massacre and handguns were effectively banned in the aftermath of the 1996 school shooting in Dunblane.

Australia introduced sweeping new gun laws after 35 people in Port Arthur, Tasmania, were shot dead in 1996. Finland - which has some of Europe's most relaxed firearms legislation and highest rates of gun ownership - placed extra restrictions on handgun permits in the wake of a 2008 college shooting which killed 11.

But not all such atrocities have provoked this response. Norway, where firearms restrictions were already robust, did not tighten its gun laws after Anders Behring Breivik's 2011 attacks.

Certainly, in the US, earlier mass shootings - there have been 13 such attacks in 2012, according to the Washington Post - have failed to provoke change.

The scale and nature of the Connecticut massacre, however, has put gun control back on the agenda in a way that other tragedies have not. Likewise, liberals hope the president's fresh mandate following his re-election will give him greater confidence to speak out on the subject.

While not impossible, fresh firearms legislation is far from certain, says Robert Spitzer, a professor at the State University of New York and author of The Politics of Gun Control.

"People are genuinely shocked by this," he adds. "Obama is in a position to exert some leadership on this issue. But it's very difficult for me to imagine the new Congress enacting new gun laws."

Even if the president were to take the initiative, he says, institutional barriers would stand in his way.

The US system means that most gun legislation is set by states rather than the federal government. Connecticut has relatively tight firearms restrictions by US standards.

A whole range of legal loopholes would have to be unpicked, too. The Brady Act, signed by President Bill Clinton in 1993, requires federal background checks on firearm purchases by flagging up buyers with a criminal record or a history of mental health problems.

But 40% of gun sales are not affected by the legislation because they take place between private individuals - including at gun show stands or through the internet.

Even where checks are performed, they are not foolproof.

Jared Loughner, who wounded US congresswoman Gabrielle Giffords and killed six others during a 2011 shooting in Arizona, was sold a Glock 19 handgun despite having had several run-ins with the police and having been thrown out of his college for erratic behaviour. But he had never been convicted of a crime nor assessed by mental health professionals.

In response, supporters of gun rights argue that clinical outreach, not new regulations, is the answer.

"I think this is more of a mental health problem than a gun control problem," says Mr Jacobs.

Advocates of greater regulation point to the fact that the US has 3.2 firearms homicides per 100,000 population compared with 1.6 for Canada, 1.0 for Australia and 0.1 for England and Wales, according to a 2012 report by the United Nations Office on Drugs and Crime.

But even the staunchest gun control supporters concede it may be impossible to ever envisage UK-style gun laws in a nation where there is currently nearly one firearm for every citizen.

Ultimately, says Ms Goss, any political pressure for change will have to come from the bottom up rather than the top down.

"I don't think leaders are going to lead on this," she says. "I think they are going to follow."