How smooth should a chamber wall finish be? How rough does it have to be before it starts causing extraction problems in a high pressure cartridge?

400 grit with oil on the paper.

How do you polish a chamber that is too rough?

Gonhuntin--I use a case drilled out through base primer for a 1/4 in bolt or threaded rod for length if doing it in action.Use it in a drill.
Not real fast, and steady hold on center.Few seconds at a time.Using a real fine tooth blade
Split case lengthwise,with hacksaw/Put 400 paper
in split sticking out both sides, oil and smooth it out.If straight case use as is. If bottleneck
case, cut it off 1/8 inch back of shoulder.After
every try, wipe out oil and look down barrel and
chamber at a light source, and you should see
it getting smoother.As you go check it with fired unsized brass.Ed.