Dudechevy,
While it is correct that 0.3301" is the SAAMI drawing neck OD at the mouth (0.003" thicker at the neck), it is easy to forget that SAAMI uses the practice of giving a critical dimension (a limit) with unilateral tolerance in the safe direction away from that critical value. Thus, cartridge numbers are maximums with a minus tolerance because it is critical that the finished item fit in a chamber, and chamber numbers are minimums with a plus tolerance for that same reason. In other words, the printed numbers are the biggest you can make a cartridge and the smallest you can make a chamber and expect them to be able to work together.
For the 30-30 Win cartridge neck and other diameters the tolerance is -0.008". Most manufacturers aim at about the middle of the range so their slightly over or undersized output still meets the standard. In other words, their target diameter value will be the drawing number -0.004". For the 30-30 case mouth, loaded with a bullet under it, 0.3261" would be the average and the expected most common value. Some makers may go a bit thicker for brass strength, but it can also be as thin as 0.3221" and still meet the standard. A typical resized case will be 0.001"-0.003" smaller after being drawn over the expander to produce the interference fit that holds onto the bullet. Because a resizing die has to work with the thinnest brass the standard allows for, you would expect a sizing die to bring the OD down to about 0.317"-0.319" before the expander sets the final ID. However, if the neck is not minimally thick, one would expect the expander to bring it out to a larger diameter than you are measuring.
The above makes me wonder if you have the correct expander in your die. You can remove it and measure it. We would expect to see it about 0.306" at the widest place.
I think you may be getting a measuring error on the inside of the case mouth. If you are using calipers, it's pretty common for the ID jaws, which have small flats on them, to report case mouth ID's about 0.002-0.003" undersize. They work well measuring between two inner flat surfaces, but not for small round holes. You need pin gauges or a small hole gauge to make a transfer measurement.
A better approach with calipers (a micrometer is better for this, and is what I usually recommend for this kind of measurement;
this outfit has a basic one for $17) is to stick to O.D.'s. Measure the outside of a bullet all around and take your average result. The bullets are usually 0.308-0.3085", but we need to calibrate your calipers. Even if the absolute measurement is off by a thousandth, the difference should be pretty close to correct. Next, make the same measurement of the resized neck all around and average your result for that, too. Finally, seat the bullet you measured and measure the neck OD over top of the bullet bearing surface.
This information will tell us how thick your case necks are.
One other thing that occurs to me is that you should try resizing one case with the expander removed. Let's make sure the expander isn't too high up in the die. That will also increase sizing effort substantially.
