To start welcome to the site.
Neck turning is one of them things that doesn't need done. The only time I would recamend turning a neck would be to fit a tight neck chamber, or just a very light cleanup to fix consintrisity problems. Things to remember is a SAAMI chamber will have a large neck in so that it will be safe with all brass, and cutting brass only makes that brass smaller, and makes it work harder. (The brass has to move more to fill the chamber, and then sized back down to hold the bullet, working brass will make it hard.)
Most bushing dies, will not size the entire neck portion of the brass. To clarify what a donut is, it would be a bulge in the inside or outside of the case neck after firing. If you have a bulge from sizing the case it would be do to the sizing process. If I am reading what you have posted, you are over sizing the brass, and this is what is giving you the bulge. A term for this is creating a false shoulder. (This is done on rounds that will be stretched to blown out shoulders, like a 6 dasher, or AI chamber.)
General rule about neck sizing bushing is you take the thickness of your brass multiply it by two, then add the bullet diameter. Once you have this measurement you subtract .002-.004 off and that will give you your neck bushing diameter. For example if your brass measures at .010 you take .010 x 2 and get .020 you would then take that .020 and add it to the bullet diameter or .020 + .224 = .244 this number is what your loaded round should measure. Most people like about .002 neck tension on a bullet, it will give a firm hold that you can not push the bullet back into the case, when pushing it against a bench. With that said if your loaded round measures .244, then your neck bushing needs to be a .242. (.244-.002= .242)
Getting into donuts, donuts usually appear when a case is changed from it parent cartridge size. For example to make a 30BR you would size up a 6mm Br, to a 30 caliber. There will be a donut formed because what was once the shoulder is now part of the neck. With the 30BR you would then cut the donut out, and most times people use a Robinette reamer to chamber the barrel so they have a .330 tight neck chamber, so now they need to cut the brass from its original .012 size down to .0098 thickness. Because of the tight neck they cut the donut out when they neck turn the brass. Donuts also form when you neck brass down, like a 22BR. A 22BR is made from 6mm BR brass, and is necked down to 22 cal. Again a tight neck .250 chamber with .012 thickness brass the donut is cut out when the brass is cut to the chamber. I usually firform my brass before I cut the necks, and this blows the donut out. The other way a donut is formed is when a piece of brass is fired repeatably with the same bullet, at the same seating depth. It takes many firing to get this to happen. Then this donut can be cut out with an inside neck reamer. Again done unsized after the case is fired.
Getting into neck trimming, the cutting mandrel of the cutter should be mated to an expanding die. the brass should not be re-sized, but expanded so that the neck fits the mandrel.
With that all said: Do you have a tight neck chamber?
What is the size of your loaded rounds? (Thickness of the neck and bullet from one side to the other measured with calipers.)
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