27 C.F.R. § 478.116
The Director shall permit the conditional importation or bringing into the United States or any possession thereof of any firearm, firearm barrel, or ammunition for the purpose of examining and testing the firearm, firearm barrel, or ammunition in connection with making a determination as to whether the importation or bringing in of such firearm, firearm barrel, or ammunition will be authorized under this part. An application on ATF Form 6 for such conditional importation shall be filed, in duplicate, with the Director. The Director may impose conditions upon any importation under this section including a requirement that the firearm, firearm barrel, ammunition, or ammunition be shipped directly from Customs custody to the Director and that the person importing or bringing in the firearm, firearm barrel, or ammunition must agree to either export the firearm, firearm barrel, or ammunition or destroy same if a determination is made that the firearm, firearm barrel, or ammunition may not be imported or brought in under this part. A firearm, firearm barrel, or ammunition imported or brought into the United States or any possession thereof under the provisions of this section shall be released from Customs custody upon the payment of customs duties, if applicable, and in the manner prescribed in the conditional authorization issued by the Director.
27 C.F.R. §478.116