An "un-sized" portion of the rear of the neck is not necessarily a doughnut.
If you use the technique I suggested in an earlier post, resizing the case in an expander-ball die, the doughnut will show up on the outside of the base of the neck, and be measurably larger than the rest of the neck. Setting the neck-turner to just skim most of the neck will eliminate the doughnut if you just turn the very top-end of the shoulder.
This is sound advice, however if you are one of those that is really fastidious, you may feel the need to anneal the brass before this process.Theoretically less spring back going over the mandrel and more doughnut to the outside. I say theoretically because I have not actually measured the difference...... but fairly certain the difference would be measurable.
