Gunwolf,
In the thread you cited,someone said they were different because he fireformed a 7x57 case in a 7x57R chamber and it wouldn't chamber in a 7x57 rifle. It is also possible that if he fireformed a 7x57 in a 7x57 rifle that it wouldn't chamber in a different 7x57 rifle(ie even if you took the "R" out of the equation). Of course if it was fireformed in the smallest chamber first, then it would chamber in the largest.In a set of headspace gages for rimless cartridge,the difference between go and no go will be from .003"to .006", depending upon the length of the cartridge(ie 308-.003; 30-06 -.006").Therefore, the chambers of two rifles can both be within spec and a case formed in one may or may not chamber in the other. Are we having fun yet?
Mike