Replies

They are using the International Traffic in Arms Regulations, which govern export of arms from the US. Per the munitions list, export of regular firearms is regulated. Also in that same document is:

(i) Technical data (as defined in § 120.10 of this subchapter) and defense services (as de- fined in § 120.9 of this subchapter) directly re- lated to the defense articles enumerated in paragraphs (a) through (h) of this category. Technical data directly related to the manu- facture or production of any defense articles enumerated elsewhere in this category that are designated as Significant Military Equip- ment (SME) shall itself be designated SME.

Since the Internet is global, putting manufacturing instructions on the net is an "export" of "Technical data ... directly related to the defense articles enumerated in paragraphs (a) through (h)", per their interpretation of the regulations.

They used the same approach to try to stop dissemination of PGP encryption some years ago.