Correct, most seating dies are not hardened. I have used a regular chamber reamer to open seater dies from Redding many times. When I was looking for a faster and less expensive seater solution for wildcats, a Redding tech told me the sliding sleeve on their comp seaters are made from 12L14. They cut easily. I recently opened a .260 Rem seater to .260AI. I turned the reamer by hand and set the depth at .125" with the GO gauge. I've done this more than a dozen times with great luck. Certainly cheaper and quicker to buy a standard comp seater and convert it.
Bob
