Here are some studies referenced on http://en.wikipedia.org/wiki/Gun_politics:
“In an extensive series of studies of large, nationally representative samples of crime incidents, criminologist Gary Kleck found that crime victims who defend themselves with guns are less likely to be injured or lose property than victims who either did not resist, or resisted without guns.
“There is no evidence that victim use of a gun for self-protection provokes offenders into attacking the defending victim or results in the offender taking the gun away and using it against the victim. Kleck has also shown, in his own national survey, and in other surveys with smaller sample sizes, that the numbers of defensive uses of guns by crime victims each year are probably substantially larger than the largest estimates of the number of crimes committed of offenders using guns. Thus, defensive gun use by victims is both effective and, relative to criminal uses, frequent.
“The economist John Lott, in his book More Guns, Less Crime, states that laws which make it easier for law-abiding citizens to get a permit to carry a gun in public places, cause reductions in crime. Lott's results suggest that allowing law-abiding citizens to carry concealed firearms deters crime because potential criminals do not know who may or may not be carrying a firearm. Lott's data came from the FBI's crime statistics from all 3,054 US counties.
“Among the 15 states with the highest homicide rates, 10 have restrictive or very restrictive gun laws.
“The National Center for Policy Analysis, a non-profit conservative think tank, reported the following statistics:
--New Jersey adopted what sponsors described as "the most stringent gun law" in the nation in 1966; two years later the murder rate was up 46% and the reported robbery rate had nearly doubled.
--In 1968, Hawaii imposed a series of increasingly harsh measures and its murder rate tripled from a low of 2.4 per 100,000 in 1968 to 7.2 by 1977.
--In 1976, Washington, D.C., enacted one of the most restrictive gun control laws in the nation. Since then, the city's murder rate has risen 134% while the national murder rate has dropped 2%.