[Unclenick]

44AMP has the key. It's tolerances.

Most folks are familiar with common plus or minus tolerances, but those are not what SAAMI uses. SAAMI uses unilateral tolerances, meaning tolerances that are only in one direction away from a given number. This is a common engineering practice where the number given represents a critical limit and going in its direction away from the mean would prevent functioning altogether, such as a chamber to small for you close the bolt on a maximum size cartridge case, or a case too large for you to be able to close the bolt on it in a minimum chamber. So the case dimensions, with a couple of special exceptions, are all maximum case size with a minus tolerance only, while the chamber dimensions are all minimums with a plus tolerance only.

Thus, in the SAAMI drawing of the .30-30, the case diameter 0.2 inches from the head is the 0.4145" number you referenced. That is the critical maximum. On the SAAMI drawing, right under the title plate for the cartridge is a note that says all diameters have a -0.008" tolerance except where noted otherwise (such as the bullet diameter). So the correct dimension for the location you cite may be written as 0.4065"-0.4145".

For the chamber at that same location, the diameter dimension is 0.4213", which is the critical minimum, and at the bottom right, under the underlined heading, "CHAMBER", it says the tolerance for all chamber diameters is +0.0020" unless otherwise stated. So your chamber should be 0.4213"-0.4233".

Now, knowing your brass expands under pressure and springs back a thousandth or two, I think you can see where your numbers are coming from.