Some folks can rust anything

[TheARfan]

It takes serious skill to rust a Glock lol

 

[Leadeye]

In the finishing business there is a standardized artificial perspiration test. One of the tougher ones over time.

 

[Indynic]

304 is nearly impervious, as well as some of the nitrogen alloyed stainless steels.

But guns almost always use a 410 series stainless, usually 416. These are 1) highly machinable and 2) martensitic (will harden)

The sulfur that aids in machinability increases susceptibility to corrosion by forming inclusions in the steel that give places for rust to form. Furthermore, Chromium content is in the 12% neighborhood, which is the bare minimum for a steel to be considered "stainless".

Exactly right. Austentic SS like 303, 304 and 316 are nearly impervious to rust, but they are more susceptible to galling.