MIM stands for "Metal Injection Moulding".
Many internal S&W parts are now MIM including the hammer and trigger.
Basically, powdered metal is mixed with a plasticizer and injected into a mould.
When heated the plasticizer melts, the metal melts and then shrinks slightly as the metal cools.
The mould is opened and a virtually finished part is removed.
All that's needed to finish is to case harden the part so it will wear properly.
The up side is this allows the price of the gun to be held down.
Very intricate parts can be made much cheaper, and this allows doing things that normally would significantly add to the price of a gun.
The down side is, the parts just don't seem to be as strong as forged and milled parts.
A good number of people have experienced broken MIM parts, and NOT just from S&W.
Remington used MIM extractors on their 870 Express shotguns, and apparently had breakage problems.
Remington MAY have discontinued the use of MIM extractors.
The latest guns seem to have milled extractors. (NOT 100% sure about this).
Although the industry states that MIM parts are very near to forged parts strength-wise and for longevity, "real world" experience seems to indicate otherwise.
Another down side is, public perception.
People just don't like the idea of MIM parts, even in those applications where the part is as good as a more expensive forged part.
Whether we like MIM or not, we'll be seeing a lot more of it in the future, but the bright side is, it's use will help prevent the ballooning of the price of a gun.