Importation. The bringing of a firearm or ammunition into the United States; except that the bringing of a firearm or ammunition from outside the United States into a foreign-trade zone for storage pending shipment to a foreign country or subsequent importation into this country, pursuant to this part, shall not be deemed importation.
27 CFR § 478.11
When used in this part and in forms prescribed under this part, where not otherwise distinctly expressed or manifestly incompatible with the intent thereof, terms shall have the meanings ascribed in this section. Words in the plural form shall include the singular, and vice versa, and words importing the masculine gender shall include the feminine. The terms includes and including do not exclude other things not enumerated which are in the same general class or are otherwise within the scope thereof.