When you size 308 brass down to 260, part of what used to be 308 neck is now 260 SHOULDER and is thicker. Where this junction is there forms a shoulder of thicker material. When you seat a bullet it hits this shoulder of the bullet is long enough to protrude into the shoulder area. If you have a tight enough neck you can take a fired case and drop a bullet back in it which will bottom out at the shoulder. If it is too loose and slides through, you can take a bushing and size it down enough that the bullet can go in snugly with finger pressure but still sits on the donut.
What I do is chuck the K&M shell holder in the 3-jaw and spin the case. I then go in with a chucking reamer and shave that donut off. You can also buy a carbide mandrel for about $40 from K&M that will cut the donut away when you feed it in during the neck turning process.
By shaving most of the excess neck away BEFORE you neck it down you mitigate this donut problem. The donut is one of the reasons when radically case forming you have to inside neck REAM.
I have literally made thousands of 708 and 260 cases from once fired 308 Gold Medal Match cases.
