According to the FBI's unified crime report, 77.3% of justifiable homicides, defined as the killing of a felon during the commission of a felony, committed by private citizens in 2011 involved firearms. That's 201 involving a firearm out of 260 total justified homicides.
For justifiable homicides committed by law enforcement officers in 2011, the number is even higher. According to the same FBI unified crime report police used firearms in 99.2% of the justifiable homicides they were involved in, 390 of 393 to be exact.
Furthermore, the FBI's unified crime report tells us that the rate of firearms use in justified homicides has never fallen bellow 77.3% between 2007 & 2011. Similarly the rate for law enforcement officers has never been below 98.7% for that period.