Thats interesting, what cartridge and brand? Iv encountered thicker shoulders when necking up, the thicker brass pushing to the outside that needed turning off and have assumed they were all the same.
Donuts are bit of a mystery all right , I think you are correct in the junction being a point of buildup. Iv found they mostly form on the first or second firing and that relieving that area alleviates the problem for the life of the case. I tried inside turning on my 280ai but wasn't as successful.
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