The .276" area is not a donut. It is the unsized part of the neck, when using bushing dies. It has fully expanded to the chamber after 3 or more firing.
A real donut is when the neck wall is thicker near the neck shoulder junction. This happens when making a wildcat or sizing the cartridge neck down or up in diameter.
To remove a real donut, inside neck reaming is done to remove the worst of it. This is then followed by outside neck turning.
A loaded round with a larger than SAAMI neck, will increase pressure , possibly to a dangerous level.
