Comment Now: General Gun Regulations for Areas Administered By the National Park Service
The Interior Department hasn't made it easy to comment on this issue by having the U.S. Fish and Wildlife Service, not the National Park Service, collect public input. As a result, the typical comment page does not appear on the Park Service's Planning, Environment, and Public Comment web site.
However, if you visit this page you'll not only find a pdf of the proposal but also a comment form.
The public comment period runs through June 30, 2008.