18 U.S.C. § 930
(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 -
(1) the lawful performance of official duties by an officer,
agent, or employee of the United States, a State, or a political
subdivision thereof, who is authorized by law to engage in or
supervise the prevention, detection, investigation, or
prosecution of any violation of law;
(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; or
(3) the lawful carrying of firearms or other dangerous weapons
in a Federal facility incident to hunting or other lawful
purposes.[b/]
(e)(1) Except as provided in paragraph (2), whoever knowingly
possesses or causes to be present a firearm in a Federal court
facility, or attempts to do so, shall be fined under this title,
imprisoned not more than 2 years, or both.
(2) Paragraph (1) shall not apply to conduct which is described
in paragraph (1) or (2) of subsection (d).
(h) Notice of the provisions of subsections (a) and (b) shall be
posted conspicuously at each public entrance to each Federal
facility, and notice of subsection (e) shall be posted
conspicuously at each public entrance to each Federal court
facility, and no person shall be convicted of an offense under
subsection (a) or (e) with respect to a Federal facility if such
notice is not so posted at such facility, unless such person had
actual notice of subsection (a) or (e), as the case may be.[b/]
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So if the entry into the building is not "posted conspicuously" then no conviction under (a) or (e), even assuming you exemption under (d)(3) is ignored as exercising you fundamental rights is not currently accepted by the federal government as a "lawful purpose".
As far as the NPS looks at whether your CPL is required that I don't know.