Code of Federal Regulations Title 27. Alcohol, Tobacco Products and Firearms § 27.555.209 Construction of type 3 magazines

A type 3 magazine is a “day-box” or other portable magazine. It must be fire-resistant, weather-resistant, and theft-resistant. A type 3 magazine is to be constructed of not less than number 12–gauge (.1046 inches) steel, lined with at least either 1/2 -inch plywood or 1/2 -inch, Masonite-type hardboard. Doors must overlap sides by at least one inch. Hinges and hasps are to be attached by welding, riveting or bolting (nuts on inside). One steel padlock (which need not be protected by a steel hood) having at least five tumblers and a case-hardened shackle of at least 3/8 -inch diameter is sufficient for locking purposes. Explosive materials are not to be left unattended in type 3 magazines and must be removed to type 1 or 2 magazines for unattended storage.