The Dreaded Donut

The Dreaded Donut or Doughnut seems to have come up in general shooting conversations quite a lot recently, but there doesn't seem to be that much information on the internet about the subject, other than a few forum conversations, so I thought I'd write a bit about them.

Donuts form because the brass on the shoulder of a case is thicker than the brass in the neck. Donuts can be created in a couple of ways:

1. When a case is necked up more brass is required to form the larger calibre neck. This brass comes from the thicker shoulder brass. As you enlarge the neck part of the shoulder becomes the neck.
2. When a shot is fired there is always amount of brass flow from the shoulder area into the neck. This is why we have to trim our cases occasionally. This flow of brass from the shoulder to the neck could cause the formation of donuts which will get worse after each firing.

In both cases it is the thicker brass of the shoulder entering the neck that creates a donut.
 
There are two main ways of dealing with donuts. You can either internally or externally turn your necks to remove the donut or you can seat your bullets so they don't contact the donut. If you chose to remove the donuts you can use either a standard neck turning tool once the neck has been sized or an internal reamer such as the Forster Neck Reamer once the case has been fired.
 
The links below have more information on the subject:

http://home.comcast.net/~jesse99/6XC_DonutCutter.html

http://home.comcast.net/~jesse99/donuts.html

http://home.comcast.net/~jesse99/doughnuts.html

http://www.6mmbr.com/260aiforming.html