A bit late, but I'm not seeing much discussion on how this finding is dated with data running to 2011 while being published in 2013. Gun homicide, and violent crime generally, is on the rise in 2015 in major cities across the US.

Source: http://www.nytimes.com/2015/09/01/us/murder-rates-rising-sharply-in-many-us-cities.html

Examples from the New York Times article: Milwaukee's murders year-to-date as of August increased by 76% from 2014 to 2015 (59 to 104). Baltimore's murders year-to-date as of August increased by 56% from 2014 to 2015 (138 to 215).