I am such an ornery child of the 60's ... I wouldn't be able to resist demanding to know what law I was breaking.
My guess would be they'd use a trespassing charge to put you under arrest, then release you later after ruining a good chunk of your day. Cops just love people who question authority...
This document touches on the subject (link). I was in their parking lots, so I was not on public property when I was shooting.