MIM (metal injection molding) is an interesting process.
Powdered metal is mixed with a polymer; the resulting mixtured is injected into a mold, then heated (in a vacuum, I believe) until the metal has melted and the polymer is burned off. That's the simple explanation, though in practice it is far more complicated.
The part actually shrinks by the amount of space originally taken up by the polymer, so the molds are actually made oversized to compensate for the shrink.
The resulting parts come out to size, and with whatever surface finish is needed. The parts can be heat treated, welded, etc. (depending on their composition, of course.)
What is interesting about the process is its capabilities. The parts can be made in forms that would be difficult (or nearly impossible) to machine. The desired hardness can be built into the part; unfortunately, many parts seem to be ordered in hardnesses (and therefore brittleness) that are inappropriate for the use (the engineering errors I alluded to earlier.) In addition, it is possible to construct alloys that normally cannot be had in machineable form.
Downsides? There is some concern about how the parts will perform over a period of decades. The process is only suitable for relatively thin parts (say, 1/4 to 3/8 thick), though this is changing as the process advances. It's also not suitable for parts that must maintain extremely tight tolerances; while there is a lot of conflicting information, it appears that a half-thousandth is about the limit of the process.
It's also relatively expensive from a materials cost standpoint, as well as fixturing (molds.) For instance, the hammers which S&W produces in their revolvers are probably slightly more expensive than their older models. Only in very large production runs can the increased fixturing costs be recouped - but the material is always more expensive, at least at the current state of the art. Of course, cost is only one of many factors that are be weighed by the designers when choosing this process. MIM allows them to produce ready-to-use parts in very complicated forms, which apparently compensates for the increased cost.
