The stainless used in guns is not really stainless, more like stain resistant. It will still rust if mis treated badly enough. It is also magnetic which true stainless is not. It does like different grease and oil than what a blued gun does plus threads and other sliding surfaces are slightly more prone to galling. I find it slightly more difficult to machine. It's not bad, just different. Larry
Stainless steel is defined as "an iron-base alloy containing chromium." Magnetic/non-magnetic has nothing to do with one or another type being "true stainless."
There's no steel that's impervious to corrosion or 'staining.' In reality,
all stainless steels are only 'stain resistant' to one degree or another.
The difficulty in machining various stainless steels depends largely on the percentage of chromium and/or nickel in the alloy. Some, like 303, are close to mild carbon steel on the machinability index. Other types are
extremely difficult to machine and often require carbide cutting tools.
Gun manufacturers use various stainless alloys, and more than one alloy in the same stainless-based gun depending upon the mechanical requirements of specific parts. Some can be fairly soft (frame, bbl, sights, etc.), others (trigger assy, springs, etc.) require a tougher/harder alloy and/or heat treatment.
http://www.materialsengineer.com/E-Stainless-Steel.htm