18 U.S.C. § 930 : US Code - Section 930: Possession of firearms and dangerous weapons in Federal facilities
This is probably reaching out on a limb, but would this authorize military members to carry into a post office or some other federal building?(a) Except as provided in subsection (d), whoever knowingly
possesses or causes to be present a firearm or other dangerous
weapon in a Federal facility (other than a Federal court facility),
or attempts to do so, shall be fined under this title or imprisoned
not more than 1 year, or both.
(d) Subsection (a) shall not apply to -
(2) the possession of a firearm or other dangerous weapon by a
Federal official or a member of the Armed Forces if such
possession is authorized by law;