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.
I have always looked for ways to save money and gain skills. I have the firearms basics covered so I rarely buy firearms these days. But I do watch for bargains. Sometimes a little TLC with simple tools will let you bring an old gun back to life without investing a lot of money. One such find was a side by side 12 gauge marked “W. Richards” It is not from the fine English gun maker Wesley Richards, it is a cheap knock off copy probably made in Belgium about a hundred years ago...