Dillon replaced the sizing die for free, and sent me a shipping box with label to send the broken die back to their R&D department. I guess that Dillon presses the carbide sizing ring into the die and crimps it in place. Maybe the cold day contributed to the metals shrinking and making the tolerances tighter, in combination with making the die steel more brittle?.......not sure. I'm now a little gun-shy about continuing to resize if I meet unusually stiff resistance. However, I always run into these occasionally very hard to resize cases with mixed 9mm brass......never really with .40 or .45.