FishDFly and Marinekayak,
The difference you are seeing between the two rifles is more likely associated with the surface finish of the bore in the rifles than the differences in the steel alloy. Rusting is a chemical reaction and can be accelerated by activation sites such as sharp points and sharp ridges. Those that have tried to rust brown a barrel or lock plate that they have polished with a fine grit abrasive soon learn this. The rust/browning process will start much easier on a surface that was sanded with 220 grit paper that a surface that was sanded down to 600 grit or finer.
Also, once a surface starts pitting, even if they are small and shallow, it will be more prone to rusting than one that remains smooth and clean. Roughness in a bore can also trap fouling and make it more difficult to remove which in turn can contribute to more corrosion from the salts in the trapped fouling.