I figured that out and make my own on the lathe when I did an experiment to compare reloading dies for case growth and concentricity.

The thing to keep in mind is that chamber neck length does not work by itself.

It gets subtracted by how far back the shoulder case was pushed. Those who adjust their sizer die all the way down to touch the shell holder, need to trim shorter.

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The man who makes no mistakes does not usually make anything.-- Edward John Phelps