When adjusting headspace on my Dillon 550 I must also have a case in the seating die station in order to have have consistent headspace results........if not the plate tips and longer headspace results.I read several years ago that the Dillon machines size more consistently with all stations filled with a case or loaded round. This prevents the shell plate tipping as it is raised. I must confess that I have never performed any testing to verify this, but I believe it to be true. I can see the tilt of the shell plate if I watch closely when there is only one case in place.