Rationale behind 27 CFR 555.209 (ATF explosive storage)
<p><a href="https://www.law.cornell.edu/cfr/text/27/555.209" rel="nofollow noreferrer">27 CFR 555.209</a> regarding ATF regulations of the storage of explosives in type 3 magazines (temporary storage boxes), states:</p>

<blockquote>
  <p>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.</p>
</blockquote>

<p>Other magazine types have similar requirements regarding 1/2” thick material. What is the specific rationale behind this requirement? I’m not looking for answers based on pyrotechnic experience. I’m looking for official ATF records, explanations, legal notes and documents, etc. </p>

<p>You can read the rationale if you know the docket number. The trick is figuring out when the specific rule was added. An example is a similar proposed rule from <a href="https://www.osha.gov/laws-regs/federalregister/2007-04-13" rel="nofollow noreferrer">OSHA</a>. The regulations appears to have existed before the creation of Homeland Security in 2003, and was numbered §55.209. <a href="https://www.atf.gov/file/97961/download" rel="nofollow noreferrer">The rule proposed January 9 2003</a>, and the wording of the rule is changed at that point. You can read the surrounding document to extract rationale (searching for "type 3" and "inch"). The proposal is on p. 4414, and unfortunately it only says that they it's about "added security". There is previous more technical discussion of analogous changes in other rules.</p>

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