Wanna see something interesting? Compare that map to the ones on this page:
http://www.justice.gov/archive/ndic/pub ... e.htm#Map1 documenting gang violence nationally.
Now correlation isn't causation, but I think one of three things (or some combination of them) must be true:
1. Gangs and gun control happen to both be primarily urban phenomena.
2. Gang activity spurs calls for gun control, on the theory that gang-related gun violence could be curbed by gun control.
3. Gangs thrive when they can take advantage of disarmed populations.