President Barack Obama on Wednesday proposed a package of measures intended to reduce gun violence in the wake of the Newtown school massacre last month.

Some of the steps have been tried before and others are expansions of laws and policies already in place. Some face high hurdles in Congress.

Will they work? Here's a look at some of the key measures:

Ban on assault weapons

The Federal Assault Weapons ban, a provision of anti-crime legislation President Bill Clinton signed into law in 1994, outlawed military style semiautomatic weapons that fire one round per trigger pull and automatically eject the shell casing and reload the chamber.

In addition to these weapons, the ban also limited semiautomatic rifles, semiautomatic pistols and semiautomatic shotguns that can accept a detachable magazine and have at least two military style features.

Congress allowed the prohibition to expire in 2004.

Sen. Dianne Feinstein said in December that she would introduce a bill to ban assault weapons.

Did it work?: Two studies point to too little evidence or too little time having passed to calculate the impact of the ban.

A provision in the 1994 law required the attorney general to deliver a report to Congress within 30 months of the ban evaluating its effects.

The summary of that report, conducted by the National Institute of Justice, said that "the public safety benefits of the 1994 ban have not been demonstrated."

The authors of the study suggested further tests of enforcement techniques, including "strategic crackdowns on 'hotspots' for gun crime and strategic crackdowns on perpetrators of gun violence. The authors suggested these techniques might be "more immediately effective, and certainly less controversial, than regulatory approaches alone."

A June 2004 University of Pennsylvania study found that the ban succeeded in reducing crimes involving assault weapons. But the benefits at the time were outweighed by increased use of non-semiautomatic weapons, which the study said were used more frequently in crime. The researchers could not credit the ban with a drop in overall gun violence over the same period.

The study did point out that since assault weapons were used no more than "8% of gun crimes, even before the ban," its impact was likely too small to reliably measure.

High-capacity magazines

The same 1994 anti-crime bill also banned magazines that held more than 10 rounds of ammunition. But it, too, expired after 10 years.

Following a 2011 attack in Arizona that killed six people and seriously wounded former Rep. Gabrielle Giffords, congressional leaders called for a ban on high-capacity magazines. The shooter in that attack used a semi-automatic pistol with a 33-round magazine.

Sen. Frank Lautenberg, D-New Jersey, announced a proposal to limit high-capacity magazines. No legislation was enacted.

After the Sandy Hook Elementary School shooting in Newtown, Lautenberg announced plans to re-introduce legislation to ban high-capacity magazines. Other legislators are pursuing requirements for background checks on the purchase of ammunition as well as seeking to ban the online sale of ammunition.

California Rep. Mike Thompson told CNN on Tuesday that a ban on high-capacity magazines could garner Republican support, but a full-scale prohibition on assault weapons would be difficult to get through the GOP-controlled House.

Did it work?: The 2004 University of Pennsylvania study noted that guns with high-capacity magazines were used in up to 25% of gun crimes, but it was not clear how often the outcome of the attack depended on the capacity of the magazine.

The study did note that since the rate of a shooter hitting intended victims is low in gun crimes, the ability to fire more shots more quickly increases the likelihood of a target being hit.

A Washington Post analysis also found that during the10-year ban on high-capacity magazines, those seized by police in Virginia dropped during that span. When the ban was lifted in 2004, the seizures rose "sharply."

Researchers interviewed by the Post note that the ban could have helped limit the availability of the magazines. But the Post analysis also notes that the impact of the ban is "hard to measure."

Universal background checks