Theoretically, and assuming all proper fit and function, the mandrel the Collet Die forms the neck against should prevent a donut from narrowing the neck. The squeezed brass flows some, of course, as Mr. Guffey says. I don't know how much simply distributes around the neck circumference and how much squeezes back toward the shoulder. Of the portion that squeezes back, I would expect some follows the mandrel surface, forming a kind of internal crater rim around the inside base of the neck which should not hurt anything. Some other portion may end up in the neck OD at the corner with the shoulder.
I've been sizing cases in two steps, using a Redding Body Die followed by a Lee Collet Die for the neck. It results in very good neck alignment. However, I have a lot of brass and have not yet shot just one case enough times to see a clear picture of where the neck brass ends up flowing to with this sizing method. I should probably pick a case and load and shoot it all day by that method, with intermittent annealing, and then section it afterward to see what I get after 20-30 firings.
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