I used some finer than 600 and then I thought about it. In hand fitting parts 400 grit is the last step before true polishing. Removes almost no metal without really trying and gunk contaminating a chamber is considerably softer. Actually broken down 320 silicon carbide works best for me for cleaning up a chamber. When the bench is clean 400 does it.
For semiautos like the 100 extraction force is not great. Consider the area of the case that can contact the chamber, it doesn't take much gunk - carbon, surface rust, whatever - to cause problems.
Like HiredGun said in properly prepared ammo the brass springs back enough that polishing finer than 400 doesn't really add anything.
