Here's the information from the atf nfa booklet.
Storage of NFA Firearms
To avoid unauthorized transfers, registrants must store NFA firearms so that no one else has access to them. Firearms may be stored at locations other than the address on the firearm registration form, such as a safe deposit box. However, unless the registrant is a Federal firearms licensee who has paid the special (occupational) tax to import, manufacture, or deal in NFA firearms, the NFA Branch should be notified in writing of the new storage location.
http://www.atf.treas.gov/pub/nfab/index.htm
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