Don't forget the DEA, whose mission statement includes involvement with gangs. The DEA also has opportunities for agents who demonstrate the abilities to work in clandestine (undercover) operations outside of the U.S., but such assignments are even more risky to an agent's life than the average "narc," who is seen by gangs and cartels as worse than the average law enforcement officer in that he poses a greater direct threat to their profits -- the bottom line to that especially vicious and desperate variety of criminal.
In its beginning, and for many years, the FBI had jurisdiction over every area of federal law enforcement. Although that is no longer the case, the Bureau still thinks it's in the Hoover days and, for all intents and purposes, investigates everything from murder, bank robbery and kidnapping involving interstate flight to drugs, money-laundering and counter-intelligence. The last three, being more in the purview of the DEA, the Marshals Service and sundry intelligence agencies with countless initials, are not really "theirs" anymore, but it's hard to let go. As a result, the FBI is often seen as sticking its nose where it doesn't belong, or being holier-than-thou, but they do always get their man, so to speak. As a tradition, the FBI used to look only for CPAs (accountants) and lawyers, but now there are three other areas of expertise they accept: foreign language fluency (read "Arabic," "Farsi," etc.), computer technology and "diversified." You must be 23 years old upon acceptance as a Special Agent.
Don't let the words ATF stand for deceive you. By the way, there is really an E at the end: the agancy's full name is Bureau of Alcohol, Tobacco, Firearms and Explosives, and it has various fun specialties including firearms trafficking, violent crime impact teams, the National Integrated
Ballistic Information Center and the U.S. Bomb Data Center. Theirs is a dangerous job with considerable fieldwork. All of these agencies are part of the bureaucracy and therefore require hours of paperwork on a daily basis, the ATF no more than any other.
The U.S. Marshals Service for the most part hunts down escaped convicts (see Tommy Lee Jones and Harrison Ford) and Witness Protection (see re-runs of the TV series "In Plain View," filmed right here where I live in Albuquerque), although they also protect federal judges and have other dutues.
ICE, the largest investigative agency within the U.S. Dept. of Homeland Security, has about 19,000 employees in more than 400 offices worldwide. Their organization is divided into four main areas: Detention and Removal, Investigations, Intelligence and International Affairs. Anything that's part of Homeland Security, in my opinion, is bound be be over-secretive, corrupt and abusive of its powers. But that's just my opinion.
As far as firearms are concerned, the policies for basic sidearms vary from agency to agency, although the FBI has held out for revolvers for the longest time because it just doesn't want to count on any gun that might jam when you need it.
Try to be patient. If you pass the background and psych evaluations, which are rigorous, and exceed the basic requirements (because there are lots of people just like you who are fluent in multiple exotic languages or have CPAs or law degrees or are genius computer hackers), you'll get a shot at it -- no pun intended. But even then, you'll start out as the Probie and have to work your way up the long ladder.