For a 243 Winchester my understanding is that the SAAMI cartridge size spec is 1.627"-1.634", and the chamber size spec is 1.630"-1.640".

My Forster headspace go gauge is marked 1.630", yet measures 1.622" to the 0.400" datum

A new factory Federal cartridge measures 1.622" to the datum

Why the disparity? How would a max spec cartridge fit into a min spec chamber? Why does the go gauge measure less than what it is stamped, and it is less than the min chamber spec?