What is the best method for polishing a 12 gauge shotgun chamber to remove some light rust?? Thanks!

I would use a flex hone with some oil.

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I've done it by using a 12-gauge brass or nylon bore brush wrapped with steel wool and soaked in oil. You can also buy a chamber brush which is usually pretty tight in the chamber just for this purpose.
If it's really bad, cut a short piece of rod with the steel wool wrapped brush on the end and chuck it in a hand drill and work it out.

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John

I use newspaper wrapped around a brush and turned high speed with a drill.

For the chamber and forcing cone which often have plastic in them from a lot of shooting, I use a pistol cleaning rod that lost it's handle at some point.
I mount the rod in an electric drill.
Place a brass brush in the tip and then wrap that with ScotchBrite till its a somewhat snug fit in the chamber.
I usually use the Maroon or Grey colored scotchbrite,,a little less coarse than the more common green.

Some oil in the chamber and on the ScotchBrite and have at it.

It'll clean any rust from the surface easily as well as plastic fouling in the chamber and/or forcing cone.
No it won't wear the chamber bigger or out of spec. It'll polish it nicely and is only in there a few seconds overall.

I use the same thing when polishing the entire bore out after rust bluing or doing any bore work like dent removal using a 36" rod w/the end threaded for cleaning tips and brushes..

Want a higher shine,,then use the same set-up but wrap a piece of wet or dry grit paper onto the same rod lapping the edge of the paper under the end of the scotchbrite to hold it in place.
More oil to soften the paper and have it cut smoothly.
600,,800grit up to 1000+ and it's really smooth and shiny.
But any imperfections in the surface start to show up as well!

I have used the Flex Hone exclusively. It works great, get the 800 grit and the small container of Flex Hone Oil. Your chamber will shine like a new penny. Just follow the directions and you can't go wrong. I sure wish Remington would have done this to the chambers of the 870 Express shotguns!