I just cleaned up an old Ben Pearson Colt with Flitz Metal Polish and it worked very well. Used some 0000 steel wool for the real real stubborn gunk parts. It would have come off with the Flitz and some more elbow grease, but it got a little late in the evening and I got a little more hurried.
This is from the Flitz website.
PASTE METAL, PLASTIC, FIBERGLASS POLISH & PAINT RESTORER
Flitz Metal Polish is a concentrated cream that is unequaled in its ability to clean, polish, deoxidize and protect a variey of metals, painted surfaces, glass and fiberglass. Available in two different size tubes.
RonP