Gary Kleck looks at the defensive use of guns by crime victims:
Each year about 1500–2800 criminals are lawfully killed by gun-wielding American civilians in justifiable or excusable homicides, far more than are killed by police officers. There are perhaps 600,000–1 million defensive uses of guns each year, about the same as the number of crimes committed with guns. These astounding totals may be less surprising in light of the following facts. About a third of U.S. households keep a gun at least partially for defensive reasons; at any one time nearly a third of gun owners have a firearm in their home (usually a handgun) which is loaded; about a quarter of retail businesses have a gun on the premises; and perhaps 5% of U.S. adults regularly carry a gun for self-defense.Keeping a gun for home defense makes most defensive gun owners feel safer, and most also believe they are safer because they have a gun. The belief is not necessarily a delusion. People who use guns for self-protection in robberies and assaults are less likely to have the crime completed against them (in a robbery, this means losing their property), and, contrary to widespread belief, are less likely to be injured, compared to either victims who use other forms of resistance or to victims who do nothing to resist. (Criminals take the gun away from the victim in less than 1% of these incidents.) The evidence does not support the idea that nonresistance is safer than resisting with a gun.
And then there’s the deterrence effect. (From Kleck’s 1991 Point Blank.)