I've been an importer in the USAsince 1995. The rules have been the same as far as I know. Wood stocks are firearm component parts and require import licensing if valued over $100. The same goes for other metal components valued $100 or less with the exceptions being listed in the statute. Telescopes and scope mounts are exempt from licensing. Anyone importing firearm components to the USA valued over $100 without an approved Form 6 import permit are in violation of ITAR. I'd guess the shippers could be cited too if caught. Mail volume is huge so many simply don't get caught but it only takes one time to ruin your day and I've heard the stories. The same with exporting. Even shipments that are exempt from licensing must be lodged with Customs and Border Protection / U.S. Department of Census via the ACES system so there is an International Transaction Number before they can be posted legally.
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Tariff codes have nothing to do with any of it other than they must be listed on the Customs Declarations.