The expander in your sizing die is not perfectly concentric and will introduce more runout than if you don't use one, provided you use a quality die. I use a Cartensen FL button die for my 300 RUM. It has no expander on the depriming rod.
When loading a 300 RUM and like cases, you have a lot of powder and usually some air space. In order to get the load burning in the case and not in the barrel you can vary neck tension to hold the bullet longer, use a hotter primer, or seat the bullet deeper in the case. This all effects ES and accuracy. In my 300 I use RL-25, a very slow burning powder, and a Fed. 210 primer with a 30+ inch barrel. I shoot 210-220 grn bullets.
The part about sizing the case to fit the chamber is simply adjusting the die so when you close the bolt you feel slight resistance when pushing the handle down. It helps if you have a die that has been cut on the basis of your chamber dimensions. There are several out there that can do that including Hornady on their CNC machines for a reasonable cost. It is also recommended to remove the ejector spring to help you feel bolt resistance. Cast the chamber if you want to know the actual dimensions.
Clear as mud?
Last edited by dbhostler; 01-18-2008 at 07:16 AM.