Dave, I use type S only for all my calibres and have yet to come across this porblem. The donut will only form from brass flow, not sizing. The bush does not size all the way to the shoulder junction, and the step is only that, a step, not a thickening of brass. Donut will form in some cals easier than others. I looked into it and the best way to keep on top is to inside ream. I have a Forster lathe that I can get inside cutters for but have not yet needed them. You will rechamber? As KiwiGreg said you couold set it back a touch and cut the throat long enough (for your intended pill) that it will not need to be seated into the junction (where donuts may form).
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