What's a high-capacity magazine? Which guns are "military style"? Why would a person use an assault weapon?
A primer on some key terms in the debate over President Barack Obama's gun-control proposals:
There are all sorts of semi-automatics -- they can be pistols, rifles or shotguns -- and they're popular sellers. They fire a bullet each time the trigger is pulled, with no need to manually move the next round into the firing chamber. That means they can fire again as fast as a person can release and pull the trigger, so long as the gun's got more ammunition at the ready. Semi-automatic weapons are popular with hunters, sport shooters and gun enthusiasts.
The sale and manufacture of some semi-automatics deemed to be "assault weapons" was banned for a decade. That law expired in 2004.
The shooters used semi-automatic rifles in the Colorado movie theater attack in July that killed 12 people and injured 70 and in the slaying of 20 children and six adults at a Connecticut elementary school in December.
They're on the battlefield and show up in action movies, but fully automatic weapons aren't common among civilians in the United States.
While a semi-automatic can fire one bullet per trigger pull, an automatic keeps firing bullets as long as the trigger is pulled once. Full automatics range from the Prohibition-era machine guns to modern rifles, pistols or shotguns.
Sales of full automatics are restricted by federal law -- buyers need a special permit from the Bureau of Alcohol, Tobacco, Firearms and Explosives. That requires an extensive background check and paying a $200 tax. Some states and local governments prohibit private ownership of full automatics.
In 1994, Congress passed and President Bill Clinton signed into law a ban on some semi-automatic rifles and handguns that were deemed "assault weapons." Defining the term was tricky then and remains controversial today.
Under that now-expired law, some new guns were banned by name, including the Uzi, the AK-47 and the Colt AR-15, which is similar to the military's standard issue M16.
The law also covered some other semi-automatic rifles that are used with detachable magazines -- devices that hold ammunition and feed the bullets into the firing chamber automatically. Such rifles were banned only if they had two or more additional characteristics listed in the law, such as a folding stock or a pistol grip.
Guns already sold to buyers before the ban were exempt and could be resold. Meanwhile, manufacturers skirted the ban by producing similar guns under new names or making minor design changes, such as removing a bayonet mount.
Obama says he wants Congress to ban what he calls "military-style assault weapons," but he hasn't defined the term, so it's unclear which guns would be covered. He describes his plan as reinstating and strengthening the 1994 assault weapon law.
That 1994 law, however, wouldn't have covered the military-looking Bushmaster .223 rifles used in the Aurora, Colo., and Newtown, Conn., shootings, had it still been in place in 2012. The old law did apply to another aspect of those shootings -- high-capacity magazines.
Why would a person use an assault weapon? They are considered by some people to be fun to shoot; they can be used for hunting, depending on the weapon and the size of the animal; and because they resemble military rifles they can appear particularly menacing when used for personal defense or home protection.
Obama wants to reinstate the ban on sales of new high-capacity magazines, defined as those that hold more than 10 rounds of ammunition.
These magazines allow a shooter using a semi-automatic weapon like the Bushmaster .223 to fire more bullets before pausing to reload. Police said Connecticut school attack suspect Adam Lanza had several 30-round magazines with him. In the Colorado theater attack, police have said suspect James Holmes used a 100-round drum magazine.
More than a third of Americans -- 36 percent -- say someone in their household owns a gun, according to an Associated Press-GfK poll conducted Jan. 10-14.
Millions use guns for hunting, sport and target shooting. Hunting guns include an array of shotguns and rifles of various types and sizes, including semi-automatics and the traditional bolt-action rifles. The prey and a hunter's personal preference determine the weapon and the kind of ammunition used.
Some people keep guns solely for protection. Carrying a concealed handgun is legal in every state but Illinois under certain conditions; for example, the gun owner might need to pass a background check first. Some states require safety classes. Some state and local laws, including in California and New York, make it difficult if not impossible to get a license to carry a concealed weapon. Illinois law bans carrying concealed weapons, but a federal appeals court overturned that law in December. The ruling is likely to be appealed.
Federal laws prevent the government from tracking how many guns are sold every year and who buys them, so there are no definitive statistics.
Roughly 310 million guns were owned or available for sale in the United States in 2009, according to a study by the nonpartisan Congressional Research Service. That's about 114 million handguns, 110 million rifles and 86 million shotguns.
Federal law requires anyone who buys a gun from a licensed dealer to submit to a background check. Convicted criminals and people who have been declared by a judge to be "mentally defective" are among those barred from buying a gun.
The Brady Campaign to Prevent Gun Violence says as many as 40 percent of all gun sales are completed without a background check. That's because sales between private gun owners and sales at gun shows are exempt under federal law.
Only California and Rhode Island require background checks for sales between private sellers and buyers. Colorado, Illinois, New York and Oregon require background checks for all sales at gun shows. Three other states, Connecticut, Maryland and Pennsylvania, require such reviews for handgun sales.
Obama has proposed a federal law requiring universal background checks for every gun sale.
It's the Constitution's Second Amendment that guarantees the right to "keep and bear arms."
The actual text reads: "A well regulated militia being necessary to the security of a free state, the right of the people to keep and bear arms shall not be infringed." What that means has been debated and argued for decades.
The Supreme Court in 2008 ruled that Americans have a right to firearms, regardless of whether they serve in a militia. The justices also have signaled that some gun regulations could survive legal challenges, but they haven't resolved which ones are permissible.
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