"The only way to stop a bad guy with a gun is with a good guy with a gun," says Wayne LaPierre, the vice president of the National Rifle Association.

That's become the kernel of the NRA's response to recent US mass shooting tragedies (such as Wednesday's massacre of nine people in a Charleston church) — if only more people carried guns for protection, the thinking goes, then they would be less likely to be victimised by gun-wielding criminals.

The challenge to that argument is that, data shows, guns are rarely used in self-defence in the US — especially relative to the rate at which they're used in criminal homicides or suicides. A recent report from the Violence Policy Center, a gun control advocacy group, put those numbers in some perspective, and I dug up the raw numbers from the FBI's homicide data. Take a look:

In 2012, there were 8,855 criminal gun homicides in the FBI's homicide database, but only 258 gun killings by private citizens that were deemed justifiable, which the FBI defines as "the killing of a felon, during the commission of a felony, by a private citizen."

That works out to one justifiable gun death for every 34 unjustifiable gun deaths.

Or, look at it this way. Centers for Disease Control and Prevention data shows that in 2012 there were 20,666 suicides by gun. That works out to one self-defence killing for every 78 gun suicides. CDC data shows that there were more than twice as many accidental gun fatalities as justifiable killings.

There are, of course, plenty of solid arguments for robust 2nd Amendment protections. Millions of people use guns for sport and recreation every day. The vast majority of gun owners are responsible citizens, not criminals.

But, though some people certainly use guns for self-defence, the data suggests that overall, guns are used far more often for killing then self-defence. As a result, it's may be thinking twice about arguments for more guns in schools, churches and other public places.