Diameter of the carbide ring in MY Lee FCD, measures .470, this was checked at various places around the ring.
Diameter of MY cast bullets, both 190 gr. Lee semi wad cutter and 228 gr Lee round nose sized to .452. Measurement .452 at various points around the bullet.
Checked a random sample of cases for the case thickness near the mouth. These were from various manufactures. Ran from .007 to .011. Most measured .009, with .010 coming in second.
What does this prove? Any case with a thickness over .009 thickness is going to post size a lead bullet using the Lee FCD that I own and using cast bullets that I size to .452. That folks is a fact.
you really need to match the diameter of the sizing ring to the diameter of the chamber you are shooting that round in to have the post sizing ring work the way it was intended.
really the question you need to ask yourself is what is my maximum tolerance and make sure the ring is no larger.
in your case you need to use cases .009 in thickness or less or ream out the inside diameter of the sizing ring to closer match your needs.
the whole point of the sizing ring is to make sure nothing larger than the post sizing ring enters your chamber, which in theory leads to more reliability in feeding.