The NRA says the only thing that stops a bad guy with a gun is a good guy with a gun. Is that right? Here's everything you need to know:

Are guns used often in self-defense?
Not very — although the evidence on this issue is hotly disputed. National Rifle Association executive vice president Wayne LaPierre is often quoted as saying, "The only way to stop a bad guy with a gun is with a good guy with a gun." LaPierre and gun-rights advocates point to research that supports this argument, chiefly a 1994 study by Gary Kleck, a Florida State University criminologist. Based on a telephone survey of about 6,000 people, Kleck concluded that guns are used defensively to stop a range of crimes, from simple assault to burglary to rape, up to 2.5 million times a year. But other academics and statisticians have criticized Kleck's conclusions, saying he relied on firearms owners' self-reporting their defensive gun use — problematic because some respondents might have categorized aggressive, unlawful gun use as self-defense — and then extrapolated that unreliable data to cover the entire nation. Those critics point to other figures that suggest defensive gun use is actually quite rare.

What figures?
Gun skeptics note that in 2012 there were 8,855 criminal gun homicides in the FBI's database, but only 258 fatal shootings that were deemed "justifiable" — which the agency defines as "the killing of a felon, during the commission of a felony, by a private citizen." Another study by the nonpartisan Gun Violence Archive, based on FBI and Justice Department data, found that of nearly 52,000 recorded shootings in 2014, there were fewer than 1,600 verified cases where firearms were used for self-defense. Gun advocates counter that not all instances of defensive gun use are reported to the police, and that in most cases shots are never fired, because simply displaying a weapon can deter a criminal. Firearms can "ensure your or your family's personal safety," said Brian Doherty, author of Gun Control on Trial, "even if you don't actually plug some human varmint dead."

Will a gun make you safer?
Most Americans think so. According to recent Gallup polls, 63 percent of adults believe having a gun in the house will make them safer and 56 percent think the country would be safer if more people carried concealed weapons. But numerous studies suggest that owning a gun can actually increase a person's risk of bodily harm and death. Research published this year in the American Journal of Epidemiology found that the 80 million Americans who keep guns in the home were 90 percent more likely to die by homicide than Americans who don't. A paper in the American Journal of Public Health, meanwhile, determined that a person with a gun was 4.5 times more likely to be shot in an assault than someone who was unarmed.

What about home intrusions?
Having a gun close at hand might make you feel better protected against violent burglars, but in fact the annual per capita risk of death during a home invasion is 0.0000002 percent — essentially zero. On the other hand, a 2014 study from the University of California, San Francisco, shows that people with a gun in the house are three times as likely to kill themselves as non-firearm owners. More than 20,000 Americans shoot themselves to death each year, accounting for two-thirds of gun fatalities. "It's not that gun owners are more suicidal," said Catherine Barber, who heads a suicide prevention project at the Harvard School of Public Health. "It's that they're more likely to die in the event that they become suicidal, because they are using a gun."

Do armed civilians ever foil mass shootings?
Yes, but not regularly. An FBI study of 160 active-shooter events between 2000 and 2013 found seven incidents in which an armed civilian shot the gunman and ended the rampage. Only one of those involved a typical "good guy with a gun"; professionals — off-duty cops and armed security guards — fired in the six other cases. Still, "good guys" do occasionally stop shooting sprees: Earlier this year, a concealed-carry holder in Philadelphia shot a gunman who suddenly opened fire inside a packed barbershop, killing him before he took anyone else's life. "It could have been a lot worse," said police Capt. Frank Llewellyn. "He saved a lot of people in there." But generally speaking, authorities are uneasy about such civilian interventions.

Why's that?
Because most civilians don't have the skills to handle an active-shooter situation. In some states, a concealed-carry permit requires no firearms training at all. "The notion that you walk into a gun store and you're ready for game day is ridiculous," says David Chipman, who served on a SWAT team with the Bureau of Alcohol, Tobacco, Firearms, and Explosives. A recent case in Houston highlights the risks of "good guys" opening fire: A man who saw a carjacking in progress shot at the perpetrators, but missed and hit the car owner in the head. Sometimes, Chipman says, the best thing to do is not to play hero, "but instead try to be the best witness you can be."

When good guys stand down
Student and Air Force vet John Parker Jr. was legally armed and ready for action when shooter Chris Harper-Mercer went on a rampage and killed nine people at Oregon's Umpqua Community College in early October. But Parker and several other veterans on campus resisted the urge to enter the fray, fearing police would mistake them for additional shooters. "Luckily we made the choice not to get involved," Parker said, "which could have opened us up to being potential targets ourselves." Joe Zamudio, a hero in the 2011 mass shooting in Tucson that seriously wounded former Rep. Gabrielle Giffords, helped subdue gunman Jared Loughner — but not before he nearly shot an innocent man. Leaving a drug store as shots rang out, "I clicked the safety off, and I was ready," Zamudio recalled. "I had my hand on my gun [in] my jacket pocket." As he rounded a corner, Zamudio saw a man holding a gun. "And that's who I at first thought was the shooter. I told him to ‘Drop it, drop it!'" In fact, it was another man, who had wrested the gun away from Loughner. Fortunately, Zamudio held his fire. "Honestly, it was a matter of seconds," he said. "I was really lucky."