Law enforcement technology has come a long way in the same period of time, and it's paid dividends. License plate readers, coordination of records on things such as gangs and violent offenders, closed circuit cameras, etc. Also, you've got things like funding for initiatives to combat poverty and combat urban blight which are both well known breeding grounds for crime and especially violent crime. If I wanted to drift into more speculative waters, I'd even say that basic things like expanded access to cell phones has helped, where people can more easily report disturbances before they escalate to the point of violence. Likewise, since I'm feeling particularly bold, I would bet all the lawless cities people bring up (Chicago, etc) have a lack of all or some of these things. You frequently see things like ineffective leadership or law enforcement, increased poverty (which includes people with no phone, limited access or they just assume the police won't do anything anyway) in these same places, and the gun laws are tossed in as a bandaid to systemic problem. Gun regulations are useless with no or poor enforcement... in fact it has the opposite effect because people assume (rightfully) they can get away with anything. That's all beside the point, because "mass shooter" type situations (you know, like what we're here to discuss) don't follow that model at all. Just like they also don't follow the blanket "violent crime" stats people selectively decide to put out there.