27 CFR 478.104: Secure gun storage or safety device.

(a) Any person who applies to be a licensed firearms dealer must certify on ATF Form 7 (5310.12), Application for Federal Firearms License, that compatible secure gun storage or safety devices will be available at any place where firearms are sold under the license to nonlicensed individuals (subject to the exception that in any case in which a secure gun storage or safety device is temporarily unavailable because of theft, casualty, loss, consumer sales, backorders from a manufacturer, or any other similar reason beyond the control of the licensee, the dealer shall not be considered in violation of the requirement to make available such a device).

(b) Any person who applies to be a licensed firearms importer or a licensed manufacturer and will be engaged in business on the licensed premises as a dealer in the same type of firearms authorized by the license to be imported or manufactured must make the certification required under paragraph (a) of this section.

(c) Each licensee described in this section must have compatible secure gun storage or safety devices available at any place in which firearms are sold under the license to persons who are not licensees. However, such licensee shall not be considered to be in violation of this requirement if a secure gun storage or safety device is temporarily unavailable because of theft, casualty loss, consumer sales, backorders from a manufacturer, or any other similar reason beyond the control of the licensee.