JollyRoger--I'm very interested in modern gang behavior because I live in the middle of an area where they are very active and seemingly "run the town." I've observed robberies and drug-dealing in broad daylight--and so far have luckily evaded any shootings--though a kid was ambushed and shot on a sidewalk two blocks from my house an hour after I had walked my dogs by the exact spot.
They may not be combat hardened per sae (many have actually been shot)--but I've noticed they often use tactics well-adapted to the urban environment and make good use of numerical superiority. I have no proof--but I suspect that they are getting trained by professionals who are more likely connected to national drug syndicates and/or drug cartels as opposed to military/police type tacticians. They know how to do their business fast and get out before law enforcement arrives--and I believe this encourages the use of deadly force. The majority of the shootings I read about generally have no witnesses or leads (partially because of the "don't snitch" and retribution fear that pervades the community).