I have had tremendous success with a product named "Flitz Metal Polish". It contains no abrasives and is safe to use on all metal products. A little goes a long way. I have used it on blued surfaces (does not attack bluing), nickel plating, silver, gold, stainless, etc., all without any degradation and with great results. I has been around for years. I was introduced to it by my brother-in-law who collects (and shoots) nickel plated S&W pistols. His weapons all look like new, all of the time---
Good luck. I assure you it will work on powder marks on stainless, and lot of other things.
As strange as it sounds, nothing works as well and easily as "Brownells Blue Remover" for cleaning powder burns and discoloration from the front of stainless revolver cylinders & other stainless parts.
This topic has been much discussed on the S&W forum. The thing to guard against is to avoid dishing the front of the chambers which will definately affect the accuracy according to the expert revolver mechanics. The edges need to remain sharp.