There's "stainless steel" and then there's "stainless steel"...

Extractors need to be reasonably "wear-resistant" and TOUGH. Hard as glass and BRITTLE is not a good look.

The two "common" stainless steels used in the gun biz seem to be 416, which is commonly used for barrels and bodies, and then there is 440; used for stuff that needs to be somewhat harder. Also used by bladesmiths. Lots of other prospective products in between. Beware the presence of Sulphur in "stressed" stainless steels, especially rolled bars used for barrel-making. The presence of Sulphur in more than vanishingly-small traces will cause nasty grain weaknesses in stuff like 416, especially when used as a barrel steel.

Ruger are the masters of stainless steel in the firearms biz; they are also masters of Titanium casting and fabrication, a whole other level of metallurgical voodoo. Stainless steel springs are another interesting field.