http://www.atf.gov/firearms/faq/faq2.htm#r
(R30) If I import a firearm temporarily for hunting or match shooting, do I have to take it out of the United States at all? My brother-in-law is a United States citizen and a U.S. resident. Can I just import the firearm, and leave it in the United States with him until I come back to the U.S. and need the gun again?
You may not leave the firearm with your brother-in-law. The regulations only allow you to temporarily import firearms. Therefore, you must take the firearms you import back out with you when you complete your sporting activity. In fact, the import permit you receive will have a stamp on it stating that the firearm must be taken out of the United States at the conclusion of the hunting or sporting event, as well as a stamp saying the firearm may not be transferred to another person. Therefore, leaving the firearm in the United States with your brother will result in an unlawful importation.
(R31) If I can't leave the gun with a friend or relative, could I leave it in the custody of a Federal firearms licensee in the United States?
No. Because the regulations require you to take the firearms you import back out with you when you complete the sporting activity,
you cannot leave them with anyone in the United States, even a Federal firearms licensee. Also, leaving it with an FFL will violate the stamps on your permit that state the firearm must be taken out of the United States at the conclusion of the hunting or sporting event and may not be transferred to another person.
US ATF FAQs
http://www.atf.gov/firearms/faq/faq2.htm#r
