Here is a more direct link to the proposal. It gives email, fax and snail-mail instructions on how to respond.
https://www.federalregister.gov/documents/2017/12/26/2017-27898/application-of-the-definition-of-machinegun-to-bump-fire-stocks-and-other-similar-devices
You may submit comments, identified by docket number (2017R-22), by any of the following methods:
Federal eRulemaking Portal: http://www.regulations.gov.
Fax: (202) 648-9741.
Mail: Vivian Chu, Mailstop 6N-518, Office of Regulatory Affairs, Enforcement Programs and Services, Bureau of Alcohol, Tobacco, Firearms, and Explosives, 99 New York Ave. NE, Washington DC 20226. ATTN: 2017R-22.
Instructions: All submissions received must include the agency name and docket number for this advance notice of proposed rulemaking (ANRPM). All comments received will be posted without change to the Federal eRulemaking portal, http://www.regulations.gov, including any personal information provided. For detailed instructions on submitting comments and additional information on the rulemaking process, see the “Public Participation” section of the SUPPLEMENTARY INFORMATION section of this document.