The other day, I realized I didn't understand why the ATF existed. We have the Treasury to go after people with tax problems, so that covers alcohol and tobacco. We have the FBI to go after people who commit gun crimes, and the FBI can certainly trace weapons if it has a database. The FBI can chase people who misbehave with explosives. We used to have no ATF.
Seems like having a separate agency must lead to confusion and turf wars.