Outside neck turning is a “precision” case prep step that improves consistency of the case neck wall thicknesses. It can be done a little bit to clean up “high spots” and make the cases better, or full-area to make them nearly perfect. That, of course, also makes them universally thinner so your sizing apparatus might need to be dimensioned differently to maintain desired case neck inside diameter to retain adequate grip on the bullet.
Outside turned case neck,
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