As noted in Peter Nap's picture of the sign on the door, it is 18 USC 930(a) that supposedly declares the otherwise lawful carry of a firearm off limits inside a federal facility.
It's my opinion that we already have enough precedence from the SCOTUS to determine that "self defense" is well within an "otherwise lawful purpose," but as always, it's going to take that one case to settle for sure. With any luck the current case on the post office carry in Colorado will tip the tide.
See text of the law below, including section (d) which is where we are covered.
BTW: Further proof: Paragraph (e) is what (1) sets the higher penalty for courtrooms, and (2) exempts LEOs, etc from the same. It is important to note for that exemption they specifically leave out what was clearly intended to be "the rest of us" by specifically leaving out exemption #(d.3). Sounds confusing, just take a look below and you will see what I mean.
18 USC § 930 - Possession of firearms and dangerous weapons in Federal facilities
(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.
(b) Whoever, with intent that a firearm or other dangerous weapon be used in the commission of a crime, knowingly possesses or causes to be present such firearm or dangerous weapon in a Federal facility, or attempts to do so, shall be fined under this title or imprisoned not more than 5 years, or both.
(c) A person who kills any person in the course of a violation of subsection (a) or (b), or in the course of an attack on a Federal facility involving the use of a firearm or other dangerous weapon, or attempts or conspires to do such an act, shall be punished as provided in sections 1111, 1112, 1113, and 1117.
(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.
(1) Except as provided in paragraph (2), whoever knowingly possesses or causes to be present a firearm or other dangerous weapon 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).