The original S&W stainless was very close to marine grade, a magnet would not stick to it and it did not rust.....but was too hard on tooling in the manufacturing process. Today's S&W will rust.
ALL of today's stainless firearms WILL rust ! Doesn't matter who makes them.
In finishes: the ceramics are made for BBQ grills and engine headers. They are pretty tough...but when they flake off they will cut into the base metal changing the fit of gun parts....BAD ! They offer NO lubricity !
Hard Chrome is very durable IF done correctly and BAKED per mil spec to eliminate hydrogen embrittlement (cracked slides/frames)
KG Gun coat is very durable for the do it at home and follow the directions.
It is used by many professional gunsmiths.
Duracoat isn't anything but paint (really a Sherwin-Williams product). It is Not protective of the metal from rust or corrosion and offers No lubricating factors. Any solvent melts it away and makes a mess like bad make-up. Gun cleaning solvents soften it REAL fast ! But it looks pretty !
TEFLON: In the testing of all finishes available for the FBI sniper rifles finish, only one finish made the cut...ALL the others failed ! The winner was DuPont Industrial Grade Teflon-S. That is what HS Precision applies to their fine rifles. Rocky Mountain Arms is the other applicator. Marketed as bearcoat, they finish firearms direct to the public as well as Les Baer and Weatherby.
The material is NOT available direct for do it your self finishing.
PVD finishes such as IonBond and Black Diamond as well as others are very durable and very hard. Application temperatures are a little high for gunparts and many small parts can't be done as they will deform in the process. It only comes in black. Is applied in a million $$$ machine. All fitting of parts must be done prior to finishing. Can be Gold as in movie "Face Off"
NP3 is good IF you can live with the silvery color of electroless nickle with a PTFE clearcoat.
Blueing: it is controlled rust ....the old Colt royal blue was beautiful .
Done correctly, it is an art..numbers are not smeared, key is polishing !
Black oxide: That's the black on Glock slides over the Tenifer ...will show holster wear. It's like blueing without the polishing process. Blast N Dip !
Parkerizing: best used as a surface prep..NOT a finish. WW1 & WW2 Military arms were oil dipped and baked for durability. Will rust if not oiled.
Nickle: Must be applied over copper plating and still flakes off.
KRYLON FUSION: Is use throughout IRAQ and Afghanistan on M-16's and M-4's as the Pentagon is too cheap to buy tan rifles ! (NOT Recommended)
YMMV...basically you get what you pay for !