A couple of different companies make them under different names, Lead Away is one, but they work well on stainless guns. Cylinder and barrel BTW. Clean the gun, then run a patch through and see what else comes out. Do not use on blued guns as it will damage the bluing.
I had them do a partial on a long slide Colt 1911. I was very pleased with the result and they followed the instructions for what I wanted exactly. The pistol was up on their web site, but I have not checked lately. I had them leave the slide and control parts blued and put a titanium finish on...