I don't know the exact heat treat of MIM parts. What I do know is that they are hard enough to last the life of the gun. Case hardening is usually .010 to .015 deep. By the time you grind or stone that much material off, the part would be out of spec and not usable. The concern about whether a part is case hardened or through hardened seems like a moot point.
Some seem to think that a through hardened part will be too brittle. If the hardness is drawn back from max (around 62 Rc) to a lower hardness, commonly anything from 40 Rc to 56 Rc, depending on what it does, then it's not too brittle, and can last indefinitely.
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