The composition of the stainless in early guns made machining expensive as cutting bits wore out much quicker with stainless than regular 4140 steel. Subsequently, a softer stainless was used which didn't wear out the cutting bits as quickly (and drive up production costs).
Another problem associated with early stainless pistols was galling. This happened when the parts of the same hardness were used in moving parts (slide to frame). Resultingly, the gun would wear itself out at a faster rate than would a normal blue steel gun. This was resolved by using stainless of two different hardness and by application of proper lubricants.
Modernly, the stainless steel gun has proven itself as durable as its blued steel counterpart. While it can still rust, the real advantage of stainless is that it never needs to be reblued.
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