I do the usual tune up of the newer, MIM internal parts.
The rebound slide is coated with what looks Teflon, and I don't get carried away stoning that. Anything that slides or rubs gets stoned.

I've asked the question elsewhere; anyone know if the hardness of MIM parts is surface only, or homogeneous?

The only part that really makes me uneasy is the sear engagement on the trigger. The drill is to sharpen that to eliminate creep, dull it to lighten break.
Caution is the rule, but is there a difference in hardness/durability to the MIM parts?
Thanks in advance,
Moon

I didn't read all through it, but maybe this will be helpful

Exploring the Role of Hardness in Metal Injection Molding

I think it depends on the part, what its composition is, and how it is heat treated.

I've worked over many AR and 10/22 triggers and have not found the inner metal to be softer than the outside.

Buy a mom trigger grind off the surface without getting it hot and then try to drill it. If you know anyone with a hardness tester have them check it. A thin layer of hard metal will not give a true reading because the soft metal under it will give more than a piece that is hard though out. I am betting a mim part is completely hard. If I think about it I will check aim vs cash hard part next time I have a minute in the shop. Been busy getting camp trailer updated for the summer

Pratt & Whitney has a proprietary process that hardens MIM parts all the way through with precise dimensions. They make Colt's MIM parts...don't know whether that's exclusive with P&W or if other gun companies get MIM parts from them using the same processes.

I recall reading some time ago that S&W's MIM parts were case hard, meaning only the outer surface was hard, something like .010" thick. However, I read that on the internet and you know how that goes. Personally, I wouldn't want the parts hardened all the way through. A part that is case hardened can absorb shock better than one hardened completely. Those tend to break instead.

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So many S&W's, so few funds!!

Is the action on your new revolver less than desirable? If not, I would leave it as is, possibly just lightly lubricate it.

The MIM parts are harder all the way through than the previous punched flat stock parts that S&W switched to in the 1950s.

I don't know if that means that they've got a uniform hardness all the way through, though.

I very seldom find any need to do much with the sear surfaces. Plus removing even .002 off something that was 60 or more RC with a 2000 grit stone would take a lot of rubbing

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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protocalldesign.com

Early days of the MIM controversy, it was said that there are multiple grades of MIM. Is your trigger made out of aerospace MIM? Who knows, the makers and users of such stuff aren't talking.

Rockwell makes a Superficial Hardness Tester just for the purpose, it only makes a tiny indent.

True about superficial testing. But, even with them the thickness of the hard material will cause variations in the reading. You need a minimal thickness of the hard layer of .026 to check above 62RC and that is thicker than case hardening.

I have had variations of 1-2 point testing high layer count Damascus

The diamond on my Ames only makes a very tiny dent and the major load on it is 150Kg. How deep of dent does a 100Kg on the tip of a small diamond point make in the metal. You can go to the D scale and only load 100Kg on the diamond for case hardened material.

My point and Prototype Design's is that you need to do a lot of stoning or diamond filing to remove enough material to get through the hard layer in the first place. In the second place all you should ever have to do is polish of any high spots.

Barrel shanks are not hard. If you try to pen the Barrel to cylinder gap from .002 to .003 . That is just .001 with a 600 grit diamond file your going to be at it a while. If your using a 1200 grit stone and diamond plate to polish a trigger it would take you hours to get through the surface hardening on a trigger or hammer sear

PS. If you ever watch Forged in Fire and seen the brutal beatings they give steel in the 58-60 RC you wouldn't worry much about what happens to a hammer whacking a frame