The last several posts are enough to make me misfire...

Quick legal caveat about the original subject - if you carry in a place that is posted where the signs have force of law, and you subsequently have to utilize your sidearm in self defense, you will be fighting a VERY uphill battle in court. Yes, you have a much greater chance of being alive to fight this battle, but being that you could be shown to be knowingly and willingly violating the law to begin with, takes a righteous shoot and throws it into criminal territory.
If you carry in a place that is posted where signs do NOT have the force of law, and are engaged in a defensive gun use, the prosecutor will try to hang you as a" loose cannon willfully disregarding the expressed will of the property owner so you can have you own showdown." Not quite as hard to fight as the first, but if you are in a state/area where CCW is not very common, the jury may be weighted against you.
Post offices and other federal buildings are verboten. I went to a Social Security office in a private building - inside the door to that suite was armed security and a metal detector with big NO GUNS signs everywhere. Out in the hallway was AOK.
Also, Contract Post Offices are NOT federal property, like standard PO are. It's technically illegal to have your sidearm in you CAR on PO land.

This law worked so well "going postal" became a byword in the 80s...