There is NO statutory authority for banning guns from post offices. This regulation is made up with no statutory authority.
It is not right that bureaucrats can make law without congressional approval. This is not interpreting a law with congressional approval. It is flat out making a criminal law exist where none existed before.
18 USC 930 (d)(3) says Subsection (a) shall not apply to
firearms or other
dangerous weapons in a
Federal facility incident to hunting or other lawful purposes. Now hunting is out inside the Post Office, but if carrying on a state license for self defense purposes is not "other lawful purposes," then the English language has ceased to have meaning.
Regulators should not be permitted merely to make things up and then impose them upon persons as a crime, specifically when Congress says "other lawful purposes" is a complete defense to the crime.
This is an abuse of regulation.
Good luck finding a federal judge who agrees with me (you won't). This is an insurmountable obstacle.
Note I have not discussed the Second Amendment at all. This is a very simply textual analysis of the controlling statute. It says what it says. The language is very easy to understand. No special training in the law is required to to understand that the statute says it shall not apply to firearms and dangerous weapons possessed at federal facilities for hunting or other lawful purposes. If you ain't there to rob the post office, then your purpose is probably lawful.