Stainless steel guns are very difficult to photograph, so it's hard to tell you exactly how to proceed without seeing it in person. However, it looks to me as though the finish might need something a shade more aggressive than a polish such as Flitz or Mother's. The above suggestions of using a Scothbrite pads sound like the way to go. Keep in mind that they come in different "grades" like sandpaper. It's a little confusing as 3M makes pads in at least 3 shades of gray (not 50, thank goodness). The Light Gray (7448) is ultra fine, the finest of the gray pads, and Dark Gray (6448) is Light Duty and slightly more aggressive than the Light Gray. I would suggest going conservative with the Light Gray and see if that does the trick. If not, the Dark Gray would be the most aggressive I would go with. The trick here is to go SLOW, just a few strokes at a time, and check your work often. Go in the same direction as the buffing grain; pulling or pushing only. In other words, don't just start scrubbing away like you're going after a dirty frying pan. Also, avoid the Gray (7446) Blending, as it is far too rough, about 120-150 grit equivalent. There are definitely rewards for taking your time. Finally, though it's not required, before and after pics would be greatly appreciated.