City of God. It's in Brazilian Portugese, with english subtitles, but it is so fucking good. This gangster movie makes the godfather look like crap. It's not a movie lots of people know about, but it's from the same guy that directed The Constant Gardener.
The DVD even has a documentary about the research that they did when writing the script, and that is one of the best documentaries I've ever seen. They go to the warehouse in Brazil where the confiscated guns are kept. It's like the Library of Congress for guns. They also talk to some gangsters, dealers, police officers, kids who are in detention centers for gang related crime, the chief of police and a bunch of other people.
GO BUY THE DVD