When dealing with a wildcat caliber you have to reform a standard case to fit your own. Doughnuts or a ring of thicker material occurs down in the neck when reforming a case so that some of the material from the original case forms the new neck. Thats because the body of a case have thicker material than the neck.
The doughnut is not limited to wildcat cases. Depending on brass flow and resizing methods it can occur in any cartridge.
The easiest way to handle it is to ensure the driving band (rearmost portion of bullet that is at bore diameter) is at or above the doughnut region. This region is the neck / body junction. If your chamber (and magazine in a field arm) will accommodate this you can ignore the doughnut.
If your bullet encroaches in this area, you need to inside ream the neck all the way past the neck / body junction.
Proper resizing goes a long way to minimize this. Changing brass may help as well. It’s easier to avoid it than it is to correct it.
As I sit and glass, which is common for hunters out west, I have sort of quickly discovered something I never learned when I was younger.
In my youth I had enthusiasm and spirit. In my middle to advancing years I have creaky knees, arthritis, and I tire more easily than I used to. Where I used to sit on the cold hard ground I now look for that soft spot to sooth my tired muscles. That leads me to my tip.
Carry your soft spot with you!
In my pack I carry an air filled, ultralight, camping pad....