I will tell you what worked for me, I have a Ruger 22 with the slab side barrel and I wanted to polish the flat sides of the barrel.
I bought some 1500 and 2000 grit wet-dry automotive finish sandpaper. I started with the 1500 and finished with the 2000. I tried it wet and tried it dry, dry to me worked better. I polished both flats and also the back of the bolt, and it took several hours over several days time, usually as I watched TV at night. When you first start polishing and you start getting some shine, the finish looks like a lot of small scratches, but as you keep working they will disappear.
When I finished the finish was as good as a stainless Vaquero that I have. This worked great for small flat surfaces, but a whole gun you would have a hard time getting into all the nooks and crannies, plus it would take a looong time.
I have worked around a machine shop and the machinest would take a stainless steel nut and grind off all the flats but one and shape it into a ring. Then they would take hours and hours polishing it up using sandpaper. When they were finished you could not tell their rings from a store bought silver ring.