The centers for disease control track death statistics.
Here's their report for 2009:
It contains the following tidbit of information:
So almost 60% of those deaths were suicides. Not mass killings.Firearm—In 2009, 31,347 persons died from firearm injuries in the United States (Tables 18 and 19), accounting for 17.7% of all injury deaths that year. The two major component causes of all firearm injury deaths in 2009 were suicide (59.8%) and homicide (36.7%). Firearm injuries (all intents) decreased 1.9% from 2008 to 2009. The age-adjusted death rate for firearm suicide did not change from 2008, whereas the death rate for firearm homicide decreased 5.0% in 2009 from 2008.
Of the rest, a certain number resulted from lawful action of the police.
Sort of puts the numbers into a different perspective.