Re: 308 reamer?
Thorax- Redding and others use 12L15 for their die blanks it is much softer than 4140 or 416SS in barrel blanks, they do however surface harden it to 60 RC or so, once you break through the surface hardening with a carbide boring bar it machines easily
Outsidelookinin is right normally a die reamer is smaller than a chamber reamer,David Kiff will make you a matched pair of reamers but the cost goes up and it may be hard to find someone who can heat treat your die blanks without warpage....
Jim Carsten will also make a die from your chamber reamer (he achieves sizing by making a two piece die with a press fit between the die body and the "chamber" that effectively "shrinks" the chamber dimensions