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39 USC 410 exempts Post Offices from 18 USC 930 (being a statute dealing with Federal facilities in general.) 39 USC 410, specifically dealing with post offices, states:

§ 410. Application of other laws

Release date: 2003-06-24

(a) Except as provided by subsection (b) of this section, and except as otherwise provided in this title or insofar as such laws remain in force as rules or regulations of the Postal Service, no Federal law dealing with public or Federal contracts, property, works, officers, employees, budgets, or funds, including the provisions of chapters 5 and 7 of title 5, shall apply to the exercise of the powers of the Postal Service.

(b) The following provisions shall apply to the Postal Service:

(1) section 552 (public information), section 552a (records about individuals), section 552b (open meetings), section 3102 (employment of personal assistants for blind, deaf, or otherwise handicapped employees), section 3110 (restrictions on employment of relatives), section 3333 and chapters 72 (antidiscrimination; right to petition Congress) and 73 (suitability, security, and conduct of employees), section 5520 (withholding city income or employment taxes), and section 5532 (dual pay) of title 5, except that no regulation issued under such chapters or section shall apply to the Postal Service unless expressly made applicable;

(2) all provisions of title 18 dealing with the Postal Service, the mails, and officers or employees of the Government of the United States;

Then look at 39 CFR 232.1, which clearly does prohibit guns in post offices. In pertinent part, it states:

(l) Weapons and explosives. No person while on postal property may carry firearms, other dangerous or deadly weapons, or explosives, either openly or concealed, or store the same on postal property, except for official purposes.