Fortunately, these issues don’t come up often in my practice, but the information at the Web site of the Bureau Of Alcohol, Firearms, Tobacco & Explosives can be very helpful.
The official description is: The Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) is a law enforcement agency within the U.S. Department of Justice. Its unique responsibilities include protecting the public and reducing violent crime. ATF enforces the Federal laws and regulations relating to alcohol and tobacco diversion, firearms, explosives, and arson.