I think he'd have a hard time using FOPA as a defense.
The two possibilities I see are that he's fishing for a lawsuit, or he's just foolish. Neither bodes well.
As far as a lawsuit, it's highly unlikely he'd prevail, since he was committing a (in that locale) criminal act. I really hope he doesn't expect the SAF or NRA to come running to his defense: they're busy trying to bring good lawsuits that actually have a chance of winning.
Sometimes it’s nice not to destroy the world for a change.