If you want to read it, here is the legislation that explicitly states online posting of plans for 3D-printed firearms requires a license under the Export Administration Regulations issued by the Bureau of Industry and Security. Note the section where it says ANY file in ANY format is included in the legislation and that you can receive up to 20 years imprisonment and $1 million in fines.

https://www.bis.doc.gov/index.php/documents/policy-guidance/2572-faqs-for-the-commerce-category-i-iii-firearms-rule-posted-on-bis-website-7-7-20/file

Because you probably won't read all 69 (nice) pages here relevant information under the section of "3D Printing of Firearms"

Under section 734.7(c), such “technology” or “software” may not be posted on the Internet without authorization from the Department of Commerce’s Bureau of Industry and Security (BIS).

Q.34: I am planning to post technology or software that meets the criteria in section 734.7(c). Do I require a license or other prior approval from BIS before posting the “technology” or “software” on the Internet?

A.34: Yes, a BIS license is required under the Export Administration Regulations (EAR) prior to posting on the Internet of “technology” or “software” that meets the criteria under section 734.7(c). No EAR license exceptions are available for such postings.

Q.35: I understand that section 734.7(c) applies to Computer Aided Manufacturing (CAM) files, such as in G-code or AMF format, as executable code to produce the items described in paragraph (c). However, can you confirm whether the criteria in section 734.7(c) would also to apply to Computer Aided Design (CAD) files?

A.35: Section 734.7(c) covers “technology” and “software” for the production of a firearm frame or receiver or complete firearm, controlled under ECCN 0A501, that is made available by posting on the Internet in an electronic format, such as AMF or G-code, and is ready for insertion into a computer numerically controlled machine tool, additive manufacturing equipment, or any other equipment that makes use of the ‘‘software’’ or ‘‘technology’’ to produce the firearm frame or receiver or complete firearm. Any file meeting that criteria is covered regardless of name, including CAD files. Moreover, this includes any file, including any CAD file, that can be processed by a software program into an electronic format, such as a CAM file, with no or minimal additional information or manipulation from the operator(s), and that the file once converted will be in an executable code for the production of a firearm frame or receiver or complete firearm.

Q.36: If I do not obtain a BIS license prior to posting “technology” or “software” that meets the criteria in section 734.7(c), will I be subject to penalties under the EAR? A.36: Yes. This would be a violation of the EAR and may result in significant administrative and criminal penalties under the EAR. Under the Export Control Reform Act of 2018, criminal penalties can reach 20 years imprisonment and $1 million per violation. Administrative monetary penalties can reach $308,901 per violation (subject to adjustment in accordance with U.S. law, e.g., the Federal Civil Penalties Inflation Adjustment Act Improvements Act of 2015 (Pub. L. 114 -74, sec. 701)) or twice the value of the transaction, whichever is greater. Violations of the EAR may also lead to the denial of certain export privileges, potentially for a lengthy period of time.