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Thanks lysanderxiii and AR-4C, that helps.
A couple of questions. In the diagram above, there's a body length of "1.000 (25.40) B", but it's not the full length of the body. Where does that distance begin and end, and what does "B" for basic indicate? Then there are two lengths from the base of the case to the shoulder 1.4381 and and 1.4666. Why is the second longer dimension needed, which seems to end a little beyond the start of the shoulder?
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You will notice that the 1.000" reference dimension is between two lines The "B" stands for 'basic' a reference dimension. It is there to define the case (or chamber) diameter at those planes. The first plane is 0.200" from the base of the cartridge (or bolt face) and for the cartridge the diameter at this plane should be .3759", the second plane is 1.000" from the first plane, or 1.200" from the base of the cartridge, for the cartridge, this diameter should be .3584".
The 1.4381 dimension is to the theoretical circle made by intersection of the taper of the body and the taper of the shoulder which should have a diameter of .3542". The 1.4666" dimension is to a datum circle on the shoulder cone that is exactly .3300" diameter, this is the 'headspace' dimension.
The reason for the use of a datum circle of .3300" is because there are tolerance in both the taper of the body and the taper of the shoulder which change the position of the theoretical location of their intersection, also you cannot measure to a theoretical point.