I recently found a REALLY good solvent for removing old powder. Hoppe's has a new solvent called Hoppe's Elite. It doesn't stink, and it's non-toxic, but I'll tell you what: it works. I saw metal on a couple of my modern handguns after cleaning with this stuff that I had never seen before, like the little tiny crevice where the barrel meets the cylinder on a revolver... I could NEVER get that powder fouling out of there, no matter how hard I tried. Hit it with a spray of this stuff, let it sit for 30 seconds, and wiped it with a patch, and it was like new. I have used it on black powder too, and it works great. They claim it cleans into the pores of the metal, leaving it cleaner than it came from the factory. I don't know about all that, but you might want to try it.
