The gunman who murdered nine people at an Oregon campus this week had 13 firearms, all of which were purchased legally by shooter Chris Harper Mercer or a member of his family in the last three years.

“This is a political choice that we make, to allow this to happen every few months in America,” Barack Obama said in response to the Roseburg shooting on Thursday. “Each time this happens, I am going to say that we can actually do something about it but we’re going to have to change our laws,” the president said.

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As the country mourns the lives lost, the gun control questions take center stage again. What gun control proposals could realistically be passed in the United States today, what is stopping them being enacted into law, and what effect it would have on gun violence if they were to be approved? Here are eight possibilities.

1. Close loopholes in background checks for gun sales

The proposal: Close loopholes that allow felons, perpetrators of domestic abuse, or people with a history of dangerous mental illness to purchase weapons.

Currently, federal law includes several loopholes which gun dealers can use to make legal sales without carrying out the due diligence of a background check on the person buying a gun.

If a background check takes longer than 72 hours, for instance, a gun dealer can sell the weapon without the completed check ; confusion with FBI paperwork earlier this year, for example, resulted in a delay that allowed Dylann Roof, the man who shot nine people dead in South Carolina in July, to buy a handgun that would have otherwise been denied to him.

On Friday Connecticut senators Richard Blumenthal and Chris Murphy proposed a law that would close this loophole – but others remain, including one that lets “private sellers” – family, friends, neighbors or strangers acting as unlicensed dealers – sell weapons at gun shows or in massive online markets without carrying out any background checks. House Democrat Carolyn Maloney in May reintroduced a bill to close the gun show loophole, and 18 states have increased background checks to cover some unlicensed dealers. No reliable data exists on the proportion of gun sales that are carried out by “private sellers” including gun shows.

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The problems: Congress has shown no appetite for introducing background checks – particularly since Republicans gained control of both the House and the Senate in 2014. Congress almost acted in April 2013 in the wake of the Sandy Hook school massacre in which Adam Lanza killed 20 children and six adults, a moment when voters wanted gun control more than at any point since the Columbine shooting of 1999. But the Senate rejected expanded checks , with Republican Charles Grassley arguing: “Criminals do not submit to background checks now. They will not submit to expanded background checks.”

Popular opinion, fear and money all played a part in the defeat of wider background checks in 2013 and continue to sway elected officials today. For more than two decades voters have opposed gun control measures ; the handful of Democrats who voted against background checks in 2013 were elected in states with a relatively high rate of gun ownership .

And while popular sentiment favored wider background checks at that moment, the longer-term trend of pro-gun sentiment and a fear of being labeled an opponent of the right to bear arms are likely to have convinced some senators that a vote for gun control could threaten their hopes of re-election. Also significant, given the huge cost of running for federal election in the US, is the fact that all but three of the senators who voted against the bill had received donations from pro-gun groups .

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Nor do background checks necessarily prevent gun violence. Oregon expanded its background checks in May.

2. End the ban on federal funding for research into gun violence

The proposal: Let government agencies such as the CDC research gun violence by repealing the 1996 budget amendment that says: “None of the funds made available in this title may be used, in whole or in part, to advocate or promote gun control.”

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The problem: For almost 20 years , under the control of both Democrats and Republicans, Congress has repeatedly renewed this ban – most recently in July .

The dearth of research the lack of federal funding has produced means that, despite gun control advocates’ many ideas, there is little evidence as to which would work to prevent gun violence and how well.

In 2012, the Republican who wrote the provision, Jay Dickey, urged its removal, writing in an op-ed that, unlike health researchers studying car accidents or infectious disease, “US scientists cannot answer the most basic question: what works to prevent firearm injuries?”

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Obama made a similar plea on Thursday, saying: “We spend over a trillion dollars and devote entire agencies to preventing terrorist attacks on our soil, and rightfully so. And yet we have a Congress that explicitly blocks even collecting data on how we could potentially reduce gun deaths. How could that be?”

3. Make gun trafficking a federal crime

The proposal: Make interstate gun trafficking a federal crime, and increase penalties for so-called “straw-man” sales in which someone buys a gun to deliver to a third party. The proposal had a boost this summer from the supreme court, which ruled 5-4 to uphold the federal ban on straw-man purchases even when the intended recipient would legally have been able to purchase a gun him or herself.

The problem: Traffickers frequently move guns from states with weak gun laws – many in the south and southwest, where Republicans dominate – into states with stronger regulations. Democratic senator Kirsten Gillibrand resurrected her proposal for a federal law banning gun trafficking in July, but she would need the support of pro-gun lawmakers for it to pass, because of their numbers in the Senate. Her new bill likely faces the same fate as its 2013 incarnation: death by Republican filibuster.

Regarding straw-man purchasers, the president could direct the Justice Department to prosecute offenders more aggressively.

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4. Expand the ban on sales to domestic violence offenders

The proposal: Expand the ban on the sale of firearms to those convicted of domestic violence, including abuse and stalking. Although studies disagree about the scope of the problem , nearly all concur that the mere presence of a gun in a household increases the risk of homicide; one Johns Hopkins study found that the risk increases eight-fold when the offender is a victim’s partner or relative, and 20-fold in cases where there is a history of domestic violence.

The problem: Lawmakers in 12 states, backed by billionaire former New York mayor Michael Bloomberg and former congresswoman Gabrielle Giffords, who was shot in 2011, have proposed legislation to expand a 1997 law, called the Lautenberg Amendment, which already bans some domestic violence offenders from buying guns. Opponents of new laws say that the Lautenberg Amendment works well enough, and that new legislation would be “ substantively insignificant ” in terms of its impact on gun crime, in the National Rifle Association’s words.

No national legislation has yet been proposed to Congress, where it would almost certainly face a wall of opposition from conservatives who argue against legislation perceived as extraneous, especially that thought to limit rights.

5. Public places, campuses and corporations

The proposal: Convince institutions to introduce firearm bans in the public places that they own, such as college campuses, stores and restaurants.

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The problem: Although a handful of companies, including Starbucks, Chiptole and Target , have chosen to ban guns from their premises, the rules governing college campuses and many public places are far more varied and complex . Twenty-three states let each college or university decide its own rules; 20 ban carrying a concealed weapon on campus; and seven, including Oregon, allow concealed weapons on public campuses or have a mix-and-match system of bans in different places.

The issue has repeatedly gone to the courts, which have largely sided with pro-gun state lawmakers.

6. Restore the ban on assault weapons

The proposal: Revive the 1994 ban on assault weapons, which expired in 2004.

The problem: The 1994 ban did not define “assault weapons”, instead listing 18 weapons that included specific versions of the AR-15 and AK-47 – allowing gun manufacturers wide latitude to slightly redesign weapons and avoid the ban. Most semi-automatics were allowed, weapons could easily be modified, and pre-existing assault weapons were legal to resell or own. Lawmakers could rewrite the law to close its loopholes, but under present circumstances such a bill would almost certainly fail against Republican opposition.

7. Regulate ammunition and magazines

The proposal: Ban certain types of ammunition, such as hollow-point bullets , which tend to be more lethal, and limit the number of rounds allowed per magazine – as the 1994 ban limited magazines to a maximum of 10 bullets, for instance.

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The problem: As with bans on specific weapons, proponents of broad gun rights argue that regulating ammunition would undercut the primacy of the second amendment – the right to bear arms – and that high-capacity magazines could be necessary for self-defense. The supreme court has often ruled on the side of gun rights, but this year let San Francisco laws banning hollow-point bullets and requiring owners to keep their handguns under lock and key stand. As with college campus laws, ammunition bans would almost certainly have to pass through local and state governments rather than Washington.

But, as with assault weapons, high-capacity magazines and specialized ammunition are already available in large supply around the US, meaning any regulation would likely have to be in force for decades before it affected the market and supply.

8. Waiting periods, training and registration

The proposal: Require waiting periods for gun purchases so that background checks can be finished and to encourage buyers to “ cool off ” from any violent impulses that might be motivating them to buy a weapon, require training for those wanting to be issued a gun license, and introduce a registry of weapons.

The problems: Although it could aid trafficking investigations and prevent illegal sales, a national gun registry is the great bogeyman of the NRA, the leading pro-gun organization, which joined its ideological enemies the civil-rights group the ACLU to sue the NSA over fears of such an index. Playing to fears of surveillance and the de facto criminalization of gun ownership, pro-gun groups have made a registry a non-starter in Washington.

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Training requirements vary by state , and even if Congress could pass national training requirements, legislators would be loathe to manage and unify programs across 50 states. States have decided their own waiting periods since 1998, but researchers disagree about how much the delays affect gun violence, with most finding little to no affect on homicides.

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