What’s a good metal polish to use for removing tarnish on nickel / coin finished receiver or side plates?

I generally use Flitz myself but any good polish such as Simichrome etc. will do the job. All of these are somewhat abrasive so go easy.
Jim

Is it an older gun ?

One is build in the early 50's one is from the mid 60's

Just currious I was told by a reputable gun man that on older SxS's that there was no such thing as a 'coin finish' they look coined because someone polished the faded case colors.

"NEVER DULL", works better than any other product I've ever tried.

Plated finishes should not be confused with "coin finished" steel. "Coin finished" is bare steel polished at some level. "Plated", of coures, has a thin layer of some tarnish resistant metal over the steel. Take care not to polish through the layer of plating. Take care not to bring a coin finish to an overly shiny finish. All the materials above work.