The stainless used in firearms is typically 410, or its close cousin 416. 410 is a fairly plain, low carbon steel with about 12-13% chromium. The absolute minimum Cr content to make a steel "stainless" is 11.5%. 416 is 410 with extra sulphur and manganese in it to make more machineable.
So yes, these grades barely clear the bar to be considered stainless, and they don't have the corrosion resistance of some of the higher alloys. But the other alloys usually have lower strength, and are harder to machine to close tolerances. Guns are precision-made items, so that is an important consideration.
Shotgun chokes are usually 17-4PH stainless, I have sometimes wondered how a barrel made from that would perform.