I have a 640, 629, and a 617. You get some nasty powder fouling on the front of the cylinder and the underside of the topstrap just above the forcing cone. My 617 seems to be particularly bad. Maybe its because I shoot it the most, or its because of the waxy lubricant on rimfire bullets. Whatever the case, on stainless guns, I use a good powder solvent and a bronze bristled brush. That usually does the trick. I got my brush from the local gun shop, its like a toothbrush, just with bronze bristles instead of plastic. I use Shooter's Choice solvent, though I am sure there are others that will work as well. A little chunk of 0000 steel wool soaked in solvent gets the stubborn stuff off. I have used fine steel wool (000 or 0000) soaked with oil or solvent, even on blued steel. You have to be careful and not use too much pressure, but I have removed surface rust from poorly maintained or improperly stored firearms that way without harming the blueing.
