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Not quite - the code you are referencing (18 U.S. Code § 930) is the same on postal, Federal, or USACE property where Federal employees regularly conduct business. The original post was about parks, lands, and waters controlled by USACE and under the citation authority of USACE Rangers. Buildings open up a whole different can of worms...

You found first part answer - second answer depends on which Federal district court your are in when the offense occurs. Where I worked the "forfeiture schedule" was $200 and mandatory appearance. If magistrate deems there to be other circumstances (additional counts/citations, failure to identify, criminal history, nose buggers, etc), there may be an enhancement and jail time. The USACE Ranger issues the citation at the time of the offense. If there is less than full cooperation, or there is an escalation or threat, a Ranger can request local law enforcement, game warden, or Federal law enforcement assistance and an arrest may result. Having said that, in most cases you would likely just to be asked to take the firearm off USACE property. But - that also depends on where the offense occurs - in California I would suspect the fine is way higher and an arrest is probable.