http://www.accurateshooter.com/technical-articles/reloading/o-rings-on-dies-may-reduce-run-out/O-Rings on Dies May Reduce Run-Out
Here’s an inexpensive procedure that can help you load straighter ammo, with slightly better measured concentricity (i.e. less run-out) on the case necks and bullets. Simply use a rubber O-Ring on the underside of the die locking ring. This allows the die to self-align itself (slightly) to the case that is being sized. Without the O-Ring, if the flat surface on the top of your press is not perfectly square with the thread axis, your die can end up slightly off-angle. This happens when the bottom of the locking ring butts up tight against the top of the press. The O-Ring allows the die to float slightly, and that may, in turn, reduce the amount of run-out induced during case sizing.
