AmarilloMike,

I was once told by the manager of the M21 custom shop (Bruno Pardee)that their specifications for the M21 frame called for Rockwell 42 to 48 but that they tried to keep them about 42 so they could be engraved. Anything over 37 starts to be difficult to engrave which is why most engravers bemoan the hardness of the 21. I have engraved a few for the custom shop in the 80s and early 90s 'till they closed and they took a good bit more time to cut than other guns because of the excessive tool breakage. Ditto for M70 actions.

I do not know what mild steel measures in hardness but would guess in the 20s to maybe the low 30s.

Sam Welch