From the BATF Website:
If the firearm you intend to import is an antique and was manufactured in or before 1898, you or the dealer if you choose to use one does not have to submit the Form 6 to ATF, however you must be able to prove to CBP that it was manufactured during that period. If you ship the antique be sure the package includes the required documention. CBP will accept a certificate of authenticity or bill of sale with the year the firearm was manufactured as proof of age. If the firearm was manufactured after 1898, the dealer has to submit the Form 6 to ATF for approval to import the firearm.
If the firearm is at least 100 years old or more and you can provide proof of age, the firearm will be eligible for duty-free treatment under the antique provision in the Harmonized Tariff Schedule.
For more information on antique firearms see ATF Web site.
