If you make sure that you have light feel when you close the bolt on a case and do not have to force the bolt closed. The way that this is controlled with my rifle is by how much is taken off the shoulder during turning, not enough and the bolt must be forced, creating an instant doughnut. If you get this right, the reforming of the body of the case actually draws some of the neck down into the top of the shoulder. That is why cases get shorter when fire forming improved or PPC cases. Another thing that I have see but not done is to lightly oil cases for fire forming (FROM .220 RUSSIAN TO PPC ONLY). I once saw a very good shooter doing this with a can of 3 in one oil. He would put a small drop on each case before firing and, using his finger tip distribute an even coat all over the case, and then chamber and fire it. Obviously the cases and chamber need to have all of the oil removed for subsequent use. Personally, I have never seen any proof that doughnuts harm accuracy as long as the shank of the bullet is not seated into that part of the case.