Bringing this thread back on track: always read the law, not what someone else says about the law. (Exception: if the appeals court for your jurisdiction has had something to say, you should read that too.)
Nothing bans carry in "federal buildings". The law states "federal facilities", and then defines "federal facilities" as "...a building or part thereof owned or leased by the Federal Government, where Federal employees are regularly present for the purpose of performing their official duties."
Outhouse or camp pavilion? Carry away. Building staffed by federal government employees? No-go. Ownership of the space doesn't matter; USDA meat inspectors have a designated office in each processing facility, and those designated places are "federal facilities", but the packing plant isn't.
Always start with the text of the law. In this case, it's not complicated except by those who wish to complicate it.
18 U.S. Code § 930 - Possession of firearms and dangerous weapons in Federal facilities