doughnuts are a build up of brass material at or near to the neck/shoulder junction. Causes can vary but from what I've read and witnessed......the too I hear of most are lots of neck sizing only and necking down brass to form wildcats. Easy way to check for 'em is to drop a bullet in the neck of a fired case...if it stops, you've got problems. You can also use a paper clip with a slight bend to check the area.
To get rid of 'em, you'll need to either turn necks or ream the inside of the neck. They are bad b/c if you seat a bullet deep enough in the neck, the doughnut will increase tension on the bullet to a point where pressures can increase drastically. Plus, they can't be good for run out etc...
I've seen a couple different theories on leaving them be.......Obviously, if the bullet makes contact they need to be removed. However, if the bullet isn't seated deep enough to contact the doughnut there's really no danger. On the other hand, I've also read that doughnuts may disturb the powder burn process thus creating issues in accuracy, velocity spreads and the such.
Hope that helps!
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