What are the firearms possession rules for non-U.S. citizens?

It is a class C felony for any person who is not a citizen of the United States to carry or possess any firearm, unless the person: 1) Is a lawful permanent resident; 2) has obtained a valid alien firearm license pursuant to RCW 9.41.173; or 3) meets the requirements of RCW 9.41.175.
A citizen of Canada may carry or possess any firearm so long as he/she possesses: Valid entry documentation; an approved U.S. Department of Justice ATF-6 NIA application and permit for temporary importation of firearms and ammunition by nonimmigrant aliens—if required under federal law; a valid hunting license; or an invitation to participate in a trade show or sport-shooting event being conducted in Washington or in close proximity.
A nonimmigrant alien, who is not a resident of Washington or a citizen of Canada, may carry or possess any firearm without having first obtained an alien firearm license if the nonimmigrant alien possesses: A valid passport and visa showing he/she is in the country legally; an approved U.S. Department of Justice ATF-6 NIA application and permit for temporary importation of firearms and ammunition by nonimmigrant aliens—if required under federal law; a valid hunting license; or an invitation to participate in a trade show or sport-shooting event being conducted in Washington or in close proximity.