I have checked with the BATF personally on the telephone and also the US Post Office personally on the phone (2 months ago), and it is perfectly legal to mail gun parts (including a handgun frame) for repair or lawful modification. Some Post Offices are not gun-friendly or gun-knowledgeable, and just obstinately will not accept them (particulary a handgun frame which still resembles a handgun), but it is legal to do so without an FFL.
I've heard some gunsmiths say that any serialized part of a firearm may not be shipped or mailed except through an FFL, and that any such serialized part is considered a firearm under the law. . . but that is absolutely a false rumor according to both the BATF and the USPO. :)








