Didn't they write a new law within the last year or so making it clearly illegal to carry in the PO, doing away with the whole "unlawful intent" gray area?
Possible. But was it a change to the USC or to the CFR?
See, CFRs aren't exactly laws, not when they conflict with an actual law...
South Carolina's LE Division - the people that issue permits, last I checked, said you were fine so long as you didn't go behind the counter.
Shrug.
Under Federal Law anything over 2.5" is verboten as a deadly weapon, though of course pre 9/11 you could fly with a great many common knives on your person since the palm test was the usual one with pre Federalized airport security.
Violation of the CFR, unless the penalties have changed, is a 50 dollar fine and possibly 30 days in prison. Violation of the USC ... eh, it's a year max. You'll theoretically do more time downloading movies or recording an NFL game without the express written consent of the NFL.
Various lawyers opine differently, as do various post masters. Try not to shoot anyone while you're actually at the post office. If you have to, well then you had to, and worrying about the US Attorney for your area being underworked enough to prosecute is the least of concerns.
Seperating the postal facilities from other Federal facilities is that one doesn't give a theoretical consent to a search while on the premises.
It also falls under the "not an enforcement priority" schtick currently, unless someone is making themselves a problem.
So in summary - if someone really doesn't like taking chances or is just plain too pretty for prison, then don't carry anything vaguely pointy sharp or menacing into the post office, let alone a gun. For everyone else, they can run their own cost benefit analysis and decide what they want to do.
If you're caught, try either bolting out the door or else crying a lot and pretending to be really stupid. Judges get annoyed by people that they think ought have known better, but tend to just take things as "well of course he did, he's apparently an idiot, oh well..." Like most things, fleeing the scene helps since if you're not caught red handed on postal property with what is a gun, it is whoever's word against yours as to how you had your favorite airsoft pistol with you in order to mail it to grandma and someone saw it and paniced. Your lawyer can help explain why you got scared and ran and why you later had a real firearm.