From How To Passivate Stainless Steel Parts : Modern Machine Shop

"Passivation is a post-fabrication method of maximizing the inherent corrosion resistance of the stainless alloy from which the workpiece was produced. It is not a scale removal treatment, nor is it like a coat of paint.

There is no universal agreement on the precise mechanics of how passivation works. But it is certain that a protective oxide film is present on the surface of passive stainless steel. This invisible film is considered to be extremely thin, less than 0.0000001 inch thick, which is about 1/100,000 the thickness of a human hair!

A clean, freshly machined, polished or pickled stainless steel part automatically acquires this oxide film from exposure to oxygen in the atmosphere. Under ideal conditions, this protective oxide film completely covers all surfaces of the part."

It then goes on to describe how iron can be left behind in the machining process, and that needs to be removed for passivization to occur.

Thanks, Murphy! Fascinating! I was unaware of all this metallurgical stuff. But it seems (from my reading, at least) that we're OK to polish away, as long as we're not using anything iron-based. Can't recall ever having a problem with any stainless gun I've shined up, but it's sure good to know why.