To call this a "timely study" is an understatement, but it certainly is.
[A] timely study from the Violence Policy Center (VPC) concluded that guns are rarely used for defensive purposes. According to the most recent data that’s available, there were 8,342 criminal firearm homicides by private citizens (non-law enforcement members) in 2012 — as opposed to 259 justifiable homicides. In other words, there were 32 criminal homicides for every killing of a felon who was in the process of committing a crime. And 13 states reported zero justifiable homicides that year.
Surprising no one who doesn't believe in fairy tales, Good Guys With Guns are mostly an urban myth.
It's unfortunate that we must rely on studies conducted by third parties to form a complete picture of gun violence in America. I attempted to find relevant data myself following the massacre at Sandy Hook Elementary School, but the most recent data available from the federal government is a study conducted by the Bureau of Justice Statistics between 1989 and 1992. That study found that guns are used in self-defense in approximately 1 percent of all cases.
That 20-year-old study was presented to members of Congress by the Congressional Research Service (CRS) in 2013 ahead of the eventual defeat of universal background checks.
The obsolescence of publicly available data is not a coincidence. The NRA launched a campaign in the late 90s to cripple the government's ability to collect this kind of data and, with the help of Congress, they were successful.