> If they are facing an opponent like that then call in the FBI, who generally have much better training, or ATF (no idea about their training).

Except those agencies aren't responsible for everday law enforcement, and some police departments are essentially fighting a war. For example, when I was a kid growing up in LA in the 90s, the gangs there were basically like warlords. Even recently, you can find examples of execution style gang murders in Los Angeles, and shootouts on the streets.