In his national address late Friday, President Obama promised "meaningful action" in the wake of the Newton school shooting. The statement left many wondering whether that would entail a push for new gun-control laws. If it does, the White House would have a number of options: One study estimates there are more than 20,000 laws regulating gun ownership already in place.

Advocacy groups and think tanks have worked through a number of proposals they think could reduce gun violence in the United States. Here are a few that have received the most serious consideration.

More extensive background checks. The Brady Handgun Violence Prevention Act, passed in 1993, mandated federal background checks on individual firearms from a registered firearms seller. Between 1994 and 2007, federal data show 1.6 million gun sales were blocked by background checks, half due to felony convictions.

Gun control advocates say that there's a big loophole here: Unlicensed gun sellers do not have to conduct the background checks. The Bureau of Alcohol, Tobacco and Firearms estimated in 1999 that about a quarter of the sellers at gun shows are unlicensed, noting that "some unlicensed vendors replenish and subsequently dispose of their inventories within a matter of days, often at the same show." Some studies estimate that about 40 percent of gun sales are made by unlicensed sellers.

Gun control supporters like the Brady campaign have pushed for national legislation that would require everyone to undergo such background checks. Rep. Carolyn McCarty (D-NY) introduced one bill, the Fix Gun Checks Act of 2011, that extends "Brady Act background check procedures to unlicensed transferors and transferees of firearms."

Research on whether this would reduce gun violence is, according to a CDC literature review, "inconsistent."

"Some studies indicated decreases in violence associated with restrictions, and others indicated increases," the CDC study concluded. "One study indicated a statistically significant reduction in the rate of suicide by firearms among persons aged >55 years; however, the reduction in suicide by all methods was not statistically significant."

Ban certain types of firearms. Between 1994 and 2004, the United States had a federal assault weapons ban, which prohibited the manufacturing of semi-automatic weapons for civilian use. That law had a sunset provision and lapsed during President George W. Bush's presidency. Congressional attempts to reauthorize the law have never received a floor vote.

Local governments have also tried banning specific types of firearms, most notably the District of Columbia's handgun ban, which the Supreme Court overturned in 2008 as violating the Second Amendment.

A study by the Department of Justice found that, after the ban, the share of gun crimes declined by 17 to 72 percent across the cities they studied (Baltimore, Miami, Milwaukee, Boston, St. Louis, and Anchorage). That decline, however, was largely offset by increased use of "large-capacity magazines," firearms that hold 30 or more rounds of ammunition. Those manufactured prior to 1994 were exempt to from the law.

"The failure to reduce LCM [large capacity magazine] use has likely been due to the immense stock of exempted pre-ban magazines, which has been enhanced by recent imports," the authors conclude.

An Australian gun reform law in 1996, which took pre-ban guns off the market as well, looks to have had more striking effects. Researchers in the British Medical Journal write that it was "followed by more than a decade free of fatal mass shootings, and accelerated declines in firearm deaths, particularly suicides."

Increase waiting periods. A handful of states have established waiting periods for obtaining a firearm, some lasting as long as two weeks (and some as short as two days). The idea is to create a "cool-down" period for the potential gun buyer. The federal government could, via legislation, set up a similar, national waiting period.

Researchers looked at the Brady Act's five-day waiting period, which was in place 1994 through 1998, when it was eliminated by instantaneous background checks. Publishing in the Journal of the American Medical Association, they found the waiting periods to be "associated with reductions in the firearm suicide rate for persons aged 55 years or older but not with reductions in homicide rates or overall suicide rates."

Increase public health funding. Researchers have recently begun to look at public health approaches to reducing gun violence. CureViolence, a Chicago-based non-profit, uses outreach workers to try an interrupt gun violence, much like public health workers attempt to stop the transmission of disease. Their initial work has shown some success: A recent intervention in Baltimore led to a 54 percent reduction in homicides in one of the city's most violent neighborhoods.