SAAMI chamber drawings show the distance from the rear of the chamber to the datum for headspace. They also list the diameter of the datum. That line exists in every chamber but it is an imaginary line. It can be measured and identified and its proper placement confirmed even though it is an imaginary line.
The dimension on fired brass to a point on the shoulder that is the same diameter as the datum will be closer to the cartridge base than the headspace dimension for the chamber. After firing the case rebounds back toward it's original size.
In the real world none of this matters unless you are reaming a chamber. The gun you have has a chamber and you want to try to fill it as completely as possible while making sure your loads chamber easily. Your chamber might be on the long side or it might be a SAAMI minimum chamber. The reloader simply adjusts his brass to fit the chamber. If the reloader over sizes his cases making them smaller when compared to his chamber the result can be "J" splits in the body or case head separation and shorter case life.
The dimension on fired brass to a point on the shoulder that is the same diameter as the datum will be closer to the cartridge base than the headspace dimension for the chamber. After firing the case rebounds back toward it's original size.
In the real world none of this matters unless you are reaming a chamber. The gun you have has a chamber and you want to try to fill it as completely as possible while making sure your loads chamber easily. Your chamber might be on the long side or it might be a SAAMI minimum chamber. The reloader simply adjusts his brass to fit the chamber. If the reloader over sizes his cases making them smaller when compared to his chamber the result can be "J" splits in the body or case head separation and shorter case life.