Squarebridge - usually, the chamber is cut first, then you make up at least 3 cases using whatever dies you need, then send 3 fully formed cases to a die maker. I suggest you not do this with CH4D as they would not match a chamber we had, but would only send a smaller bodied variation that was taken as standard many years ago. The shoulder of the "custom" dies sent was .006" undersize. They did not 'fix' their error when the dies were returned, yet charged again for the "repair". I will not deal with CH4D in the future.

To go at the chamber from the 'other' end, Dave Kiff of Pacific Tool and Gauge should be able to help. A google search will give his web site.