I have found another 1935 Mauser Oberndorf action that Rockwell tested at a hardness of 20. What is safe to build on this action that won't cause any future problems with the metal (binding, stretching, etc.) ??? I know it won't fly apart, but is it hard enough to build magnum calibers on ? Thanks.

A hardness of 20 on which rockwell scale-A, B, C?

Sorry about that ....... "C" ..........

The Kuhnhausen book I have suggests a shallow case hardening of .010-.015" and then an exterior hardness in the 40's range(?). Don't quote me exactly, I can't find that book right now and I'm trying to remember what it said.

20's sounds a bit light. I would agree with Blue about checking into a recarburizing service.

Or, you may wish to put this question to the gunsmithing board. I believe you may draw a more affirmative response there.