A short while ago, 39 CFR 232.1(l) prohibited firearms unless you were there for an 'official purpose'. However, 18 USC 930 basically said that the carrying of firearms into a Federal facility where state law did not prohibit it was fine as long as the behavior was incident to a lawful purpose (e.g. self defense), making it lawful to carry into a post office.
Recently, the introductory clause to 39 CFR 232.1 has changed, so this is no longer the case -- still looking for this cite.
So, here was the OLD laws:
232.1(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.
18 USC 930(p) (2) Whoever shall be found guilty of violating the rules and regulations in this section while on property under the charge and control of the Postal Service is subject to fine of not more than $50 or imprisonment of not more than 30 days, or both. Nothing contained in these rules and regulations shall be construed to abrogate any other Federal laws or regulations of any State and local laws and regulations applicable to any area in which the property is situated.
In the long run, it's best to err on the side of caution at this point and not carry in a post office.
Edit: Found this on the web:
The Post Office since 1972 has been privatized. They are not even a "quasi" federal agency anymore. They used to be called the Bureau of the Post Office or something like that when they were federalized.
So exactly how does 18 USC 930 apply to them if they are not federalized?
Finally, I found this on the net recently:
"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."
So 930 does not apply and 239 is nothing more than a rule from a privatized entity.
So, it's a private entity rule, perhaps?
The plot thickens.
Delaware Open Carry, Founder