Great, many thanks. This is exactly the how-to-proceed kind of information I've been looking for.
One other interesting bit of data from http://www.jouster.com/forums/showthread.php?41802-25-Griffin-amp-Howe-built-by-Niedner-on-o3-action:
"Many of the Niedner chambered rifles have what is called a Mann-Niedner chamber which is a very tight chamber so you must use a little caution when loading for it if that is a the case. Most Mann-Niedner chambers are so tight that the case never needs resizing."
But my rifle was barreled by Sukalle, so it shouldn't have the Mann-Niedner chamber, I think.
