FOPA makes it illegal to <i>arrest</i> a person for carrying a firearm while traveling through a jurisdiction.  It is not a defense after-the-fact, it is a complete defense.<p>If you are arrested, that is a violation of FOPA, which means a constitutional violation of your due process rights accorded under FOPA.  This means not just an automatic dismissal of the case, but significant moola for the violation of rights.