Well I'm not a metallurgist but I am a Mechanical Engineer and I still remember a bit about this stuff. Also worked with these steels a bit.
Any steel with 11% or more Chromium is considered a stainlesss steel. The Cr allows the material to form a passive Chromium Oxide barrier CrO2 that protects the surface of the metal
Wet H2 firing is not a process that I'm familiar with - but I'd imagine it involves coating the 300 series stainless with a surface "paint" of some sort and then heat treating it in a Hydrogen atmosphere (no smoking, please!
). This is either done in a Hydrogen retort furnace (an upside down bell that holds H2) or a "humpback" furnace (one designed to hold the hydrogen, which wants to rise).
The H2 is a 'reducing' atmosphere, which strips chromium oxides off of the surface of the stainless (the CrO2 is what keeps it from rusting). I'd imagine that your process results in a surface hardening (similar to case hardening or carburizing or nitriding on regular carbon steels). Whatever is placed on the surface diffuses into the top .005-.030 or so of the material, resulting in surface hardening. This is a good process for a shaft as the ductility is maintained internally so that the material is quite fatigue resistant while creating a surface hardness for the bearing running surfaces, etc.
AFAIK 300 series stainless is strictly a strain hardening material. You can buy it in differing tempers - annealed (soft), 1/4 hard, 1/2 hard, full hard, etc. The hardness is established by cold working the material. You can restore it to full softness by annealing it (heating it up to a temperature that allows all of the structure to stress relieve itself). 300 series SS is austenitic, non-magnetic, and offers superior corrosion resistance in non-acid non chloride environments, but can't be hardened all the way through by heat treating it.
To get back to the original question, 400 series SS is typically used for guns. It is a ferritic stainless (magnetic, and less rust resistant as noted above). Different alloys are chosen depending on what use it has (barrel or slide, for instance), and the method of manufacture (forging, machining, casting, cold forming, etc). Some versions are more machinable, or more forgeable, for instance. Typical alloys such as 410 and 416/416S are used. These can form martensite when heat treated and cooled (quenched) properly. This makes for a part that can be hardened after machining, although there is some risk of deformation from the heat treat process.
Some early slides were made with 17-4PH IIRC. The PH stands for precipitation hardening. It involves a supersaturated alloy solution that is cooled rapidly from annealing temperature. If kept cold the alloy stays mixed and it looks like normal alloyed metal. But heat it up a bit for the right amount of time and little crystals of "precipitant" form (typically a carbide) that induce strain in the grain structure and resultant hardness. 17-4 can get really, really hard and I'm not sure but i don't think anyone uses it anymore. Maybe for a slide but certainly not for a frame. (Now that I'm thinking about it - it seems that Caspian may have offered slides made up of the stuff). The great thing about PH material is that you can machine them soft, and then do the relatively low temp precipitation process to bring the hardness up. As the precipitation is at a pretty low temp, the risk of the part distorting is pretty much eliminated.
Hope this helps. It is pretty much about all I recall.