It is a similar situation across all borders- typically the border authorities are not required to make any special efforts to decide on an issue (i.e. what makes for a 'firearm') and should seek guidance from the proper authority (i.e. the BATF in this case). Of course, border authorities have a dual-role- both one of enforcement of the law and the facilitation of moving legal goods and such across the border. Problems can arise when someone gets it in their head they are "enforcing" a law as opposed to facilitating a legal transaction and do not seek answers from the proper authority or are given fuzzy answers from such an authority.