One side calls them "weapons of war" that have no place on America's streets. The other side says the term "assault weapons" is simply a menacing moniker designed to stir up anti-gun passions.
President Barack Obama jumped into the center of the fiery debate when he called on Congress to ban those weapons and their high-capacity magazines in the wake of December's elementary school massacre in Newtown, Conn., giving new life to the crusade of gun-control advocates to re-enact the federal assault-weapons ban that expired in 2004.
The debate begins with a simple question that has no simple answer: Just what is an assault weapon?
Lacking a rigid definition, lawmakers have struggled for years to come up with a set of criteria that sweeps in the rapid-fire, military-style rifles used in some of the nation's most sensational mass murders while leaving out popular hunting rifles that allow a sportsman to quickly fire a half-dozen bullets at a deer dashing through the forest. But it's almost impossible to do that: There are M-16 look-alikes that are far less dangerous than a common pistol, as well as hunting rifles that can do nearly the damage of Connecticut school shooter Adam Lanza's Bushmaster.
"What's politically possible to get and widely supported is not going to be effective because it'll be narrowly confined to a subset of weapons, let's say 'the scary-looking ones,''' said Gary Kleck, a Florida State University criminology professor who has written several books on gun control and gun violence.
In 1989, California lawmakers -- horrified by a mass shooting at a Stockton schoolyard -- adopted the first assault-weapons ban in the country. They simply named the guns that would be banned, and gunmakers responded by changing their names and model numbers. Since then, California and other states, as well as the proponents of a new federal ban, have incorporated a list of military-style characteristics that seem to make the most popular assault weapons especially deadly.
The seven states with assault-weapons laws have much different standards. Hawaii's and Maryland's, for example, deal only with pistols. In California, a semi-automatic rifle is deemed to be an assault weapon if it has a detachable ammunition magazine plus one of several specific features such as a pistol grip, flash suppressor or grenade launcher. Connecticut's law requires at least two of those features, which means the menacing-looking Bushmaster that killed 20 children and seven adults last month was legal. In California, it's banned.
Gun control advocates say the restrictions on military-style weapons are just common sense because those guns encourage unhinged people from imagining themselves as commandos in shopping malls and elementary schools. The other side says the wide of range of definitions shows just how arbitrary the gun bans are.
The National Rifle Association argues that people unfamiliar with guns don't understand even some of their most basic workings. They note correctly that some journalists use the terms "semi-automatic" and "automatic" interchangeably.
Semi-automatics, which feed the next round into the gun chamber as the empty cartridge just fired is ejected, require a separate pull of the trigger for each bullet. Fully automatic weapons, which keep firing as long as the trigger is held down, have essentially been banned in the U.S. since 1934.
The National Shooting Sports Foundation -- a firearms trade association based, coincidentally, in Newtown, Conn. -- says "the term 'assault weapon' was conjured up by anti-gun legislators to scare voters into thinking these firearms are something out of a horror movie." In fact, the term was introduced by the gun industry itself to boost interest in new lines of firearms.
In 1984, Guns & Ammo magazine advertised a book called "Assault Firearms."
"If you are interested in survival tactics and personal defense, we'll give you a look at the newest civilianized versions of the semi-auto submachine gun," the ad said.
Today, the AR-15-style Bushmaster and similar guns banned in California are among the most popular semi-automatic rifles in America.
"These are weapons that will shred your venison before you eat it, or go through the walls of your apartment when you're trying to defend yourself,'' said Jonathan Lowy, director of the Legal Action Project at the Brady Center to Prevent Gun Violence, arguing that they're "made for mass killing, but not useful for law-abiding citizens."
Yet gun groups say the Bushmaster works no differently than most semi-automatic firearms, and is lighter, easier to handle and hits its targets harder. Some gun owners like its looks: Bushmaster ads have boasted that owning one renews your "man card."
"The idea that the Second Amendment doesn't protect the most popular civilian rifle is absurd," said Gene Hoffman of Redwood City, cofounder and chairman of the Calguns Foundation, a gun-rights group. "Attempts to ban them are simply attempts to demonize gun owners for choosing a modern rifle."
Some gun control advocates argue that more effort should be placed on banning high-capacity magazines than arguing about what makes an assault weapon.
California's law limits detachable magazines to 10 rounds, as did the 1994 federal ban. New York this week lowered its limit from 10 to seven. But the California, New York and federal bans failed to outlaw the huge number of high-capacity magazines already in circulation.
An independent study done in 2004 on behalf of the National Institute of Justice found the share of gun crimes involving assault weapons declined significantly in six cities during the 10-year federal ban -- particularly with assault pistols, which were used more commonly in crimes because they're more easily concealed. But the study also found that the decline in assault-weapon use was offset by the steady or rising use of other guns equipped with large-capacity magazines.
Data on California's assault-weapons ban is ambiguous at best. In 2011, the state had almost the same rate of gun homicides as the nation as a whole.
Other nations have grappled with the same issues. In 1996, a gunman using semi-automatic rifles in Australia killed 35 people and wounded 23. The country reacted by banning semi-automatic and automatic rifles and shotguns. It also enacted a mandatory buyback program for all such weapons.
A 2010 study by researchers from the Australian National University and Wilfrid Laurier University found that nation's firearm homicide rate fell by 59 percent and its firearm suicide rate fell by 65 percent in the decade after the law's enactment.
The study found Australia bought back about 650,000 firearms. It's unknown how many legally owned semi-automatic rifles and semi-automatic shotguns there are in the United States, but estimates range in the tens of millions. So even major gun-control groups aren't calling for all semi-automatic rifles to be banned -- at least, not yet.
"Maybe someday," said Laura Cutilletta, senior staff attorney with the Law Center to Prevent Gun Violence, "but that would require a lot to change between now and then."
Of 10 shootings that made headlines in the United States last year, four involved AR-15-type rifles that could be affected by a proposed federal assault-weapons ban. Those four shootings, three of which occurred in December, are in bold type:
Feb. 27 -- Two students killed and three wounded at Chardon High School in Chardon, Ohio; student Thomas "TJ" Lane allegedly used a Ruger MK III .22-caliber semi-automatic handgun.
April 2 -- Seven killed and three wounded at Oikos University in Oakland; former student One Goh allegedly used a .45-caliber semi-automatic handgun.
April 6 -- Three dead and two wounded in an apparently racially motivated shooting spree in a predominately African-American section of Tulsa, Okla.; Jake England and Alvin Watts allegedly used a .38-caliber handgun of unknown make and model, according to court testimony.
May 29 -- Five killed (four in a cafe, one in a carjacking) in Seattle shooting spree; Ian Lee Stawicki used a Para-Ordnance .45-caliber semi-automatic handgun and fatally shot himself later the same day.
July 20 -- Twelve killed and 58 wounded in an Aurora, Colo., movie theater; James Holmes allegedly had a Smith & Wesson M&P15 (AR-15-style) semi-automatic rifle with a 100-round drum magazine, a Remington 870 Express Tactical shotgun, and two Glock 22 .40-caliber handguns.
Aug. 5 -- Six killed and four wounded at a Sikh temple in Oak Creek, Wisc.; Michael Page used a Springfield XD (M) 9 mm semi-automatic handgun. He killed himself with a shot to the head after being wounded by a police officer.
Sept. 27 -- Six killed and two wounded at Accent Signage Systems in Minneapolis; Andrew Engeldinger, who had just been fired from the company, used a Glock 19 9 mm semi-automatic handgun and killed himself after the rampage.
Dec. 11 -- Two people killed and one wounded at Clackamas Town Center mall in Happy Valley, Ore.; Jacob Roberts used a Bushmaster .223-caliber (AR-15-style) semi-automatic rifle, and killed himself at the scene.
Dec. 14 -- Twenty-seven killed and two wounded in Newtown, Conn.; Adam Lanza used a Bushmaster .223-caliber (AR-15-style) semi-automatic rifle but also carried a Glock 20 10 mm semi-automatic handgun. A Sig Sauer P226 9 mm semi-automatic handgun and a shotgun were found in his car after he killed himself at the scene.
Dec. 24 -- Two volunteer firefighters killed and two wounded in Webster, N.Y.; William Spengler Jr. used a Bushmaster .223-caliber (AR-15-style) semi-automatic rifle but also had a Mossberg 12-gauge shotgun and a .38-caliber Smith & Wesson revolver with him. He killed himself at the scene. Spengler's sister's remains were found in the burned rubble of his house, but the cause of her death hasn't been made public.