Lately I've been using a product called "Lucas Oil Gun Metal Polish," It is specially formulated for blued finishes, does not remove bluing, and does a nice job of getting accumulated crud and minor corrosion off the exterior of firearms. When you first rub it in, the cloth takes on a reddish color, indicating that some minor surface rust has come off. I like the stuff. I just used it on a Sako and a Tikka that I was working on, and I could easily see the improvement in appearance. It's available from the usual suspects: Brownells, Midway, Amazon, etc.