Originally Posted by kilimanjaro
It's a Federal building, the rule is the same for all of them.
No, it isn't the same. The Federal law applying to most Federal buildings has an exception for "lawful purposes." Although it hasn't been tested in court and I would not knowingly volunteer to become the test case, it has been proposed that carry for self-defense with a state-issued license/permit to carry would be covered under that exemption. Also, the law applying to most Federal buildings uses the word "facility," which is defined as "building."
Carry in a post office is prohibited under a separate regulation adopted by the USPS. It does NOT contain any exemption for "other lawful purposes," and it applies to the entire property, not just the interior of the building.