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MIM stands for Metal Injection Molding and, done right, it can produce excellent, reliable parts.

MIM entails mixing metal powder with a plastic binder to form a slurry, that can be injection molded like plastic. The "green" parts are held together only by the binder until they are sintered - heated to 1) burn off the binder and 2) solid-state weld the metal particles together.

The shrinkage has to be accounted for in initial mold design. Done right you wind up with 100% dense metal parts, that behave properly. Done wrong you get porosity and brittle garbage that certainly does not belong in a piece of emergency equipment like a pistol.

If you search the web you will find a lot on powder metallurgy and MIM. Like anything else, these things started out as cost-cutting measures and developed into mature processes over time, with use and refinement.

I hope this helps.

Regards,
Andrew