First, remember that this was while he was running for IL senate. Politics is local and in Chicago people like their gun control (At least, that's how they vote).
I notice that you used a metropolitan area, which IMO is the point of the law, to restrict access to guns in the city.
Now, I am looking at the Dem. race right now, and I noticed that one of the few voting differences between Obama and Clinton was their vote on this:
Firearm Confiscation Prohibition Amendment:
Linky.
Obama voted yes, Hillary no.