When I was working as an armored car guard, I started out with a blued Ruger Security Six. I wore it, exposed in a police style rig, ten to twelve hours a day, and climbed in and out of the truck all day in all kinds of weather.
Eventually, in an effort to ease my maintenance chores, I bought a stainless steel version of the same gun.
After about a year of the above kind of abuse, I found traces of rust in the hammer groove on the frame where rainwater had pooled.
It IS possible to make a stainless alloy that just will not rust, but such an alloy won't machine worth a damn. When you change the alloy to make it usably machinable, you have an alloy that is highly rust resistant, but not rust proof.
Rust resistance is tested by placing sample materials in a test chamber where it is sprayed with a standard saline water mist.
Ordinary polished chrome moly gun steel placed in the test chamber shows ruat after only half an hour.
When BLUED, it shows rust after an hour.
Parkerized, it shows rust after two hours.
Stainless steel gun alloy shows rust after 60 hours.
Electroless nickel and hard chrome finishes are in that ballpark. I forget which is which, but one went 50 hours and one went 70.
Robars' NP3 went 300 hours.
NP3 on a stainless steel gun should be damn near indestructable, at least from a rust point of view.
Regards
John
PS: BTW, Ayoob has speculated that, while military commandos and police SWAT officers may need dark guns, the AVERAGE citizen carrying for self defense would be better served by the high visibility of a bright gun. YMMV, of course.