I’ve heard you aren’t supposed to bottom a carbide sizing die out on a shell holder. Why?

Carbide is an extremely hard and very brittle metal. This is both an asset and a liability. The carbide’s incredible hardness and polished finish is what allows these dies to function without lubricant. However, the very hardness of the carbide is what makes it so brittle. It’s the small carbide ring embedded in the base of the sizing die that actually does the sizing. When pressure is put on it from the shell holder, it can literally shatter the carbide insert. You will need to keep a bit of clearance between the die and shell holder.