From the BATF website FAQ:
http://www.atf.treas.gov/firearms/faq/faq2.htm#b1B. UNLICENSED PERSONS
(B8) May a nonlicensee ship a firearm through the U. S.
Postal Service? [Back]
A nonlicensee may mail a shotgun or rifle to a resident of
his or her own state or to a licensee in any state. Handguns
are not mailable. A common or contract carrier must be used
to ship a handgun. A nonlicensee may not transfer any
firearm to a nonlicensed resident of another state. The
Postal Service recommends that longguns be sent by
registered mail and that no marking of any kind which would
indicate the nature of the contents be placed on the outside
of any parcel containing firearms.
(B9) May a nonlicensee ship a firearm by carrier? [Back]
A nonlicensee may ship a firearm by carrier to a resident of
his or her own state or to a licensee in any state. A common
or contract carrier must be
used to ship a handgun. In addition, Federal law requires
that the carrier be notified that the shipment contains a
firearm and prohibits common or contract
carriers from requiring or causing any label to be placed on
any package indicating that it contains a firearm. [18 U. S.
C. 922( a)( 2)( A) and 922( e), 27 CFR 178.31]
