Here is the “shiny” part of forcing cones explained in more detail than I did it.
Shotgun Report’s Technoid
(Often in error, never in doubt.)
In your recent response to a query on forcing cone finish, I am of the opinion that you are on target in the next to last paragraph, i.e. the angle of the light determines virtually all of what we see as darkness in the cone area. I have lengthened several cones myself using the “typical” 1 1/2 inch length reamer. Aligning the stones to the proper angle for honing can be a problem, but not so much any more with the “bottle brush” style hones now available from MSC, Brownell, and others. They do a nice job.
The cones cut by a standard SAAMI chambering reamer look almost like a black step at the end of the chamber, but are actually about a half inch long. Such is the effect of the light angle. As the cone gets longer, the angle gets shallower and the cone looks lighter. By the time you reach 5 inches the cone angle is very shallow (almost parallel with the barrel wall) and it reflects very brightly. With the 1 1/2 inch cones I have used, they look somewhat dark; but if you hold the barrel off angle slightly and sight down it, you can see the cone wall looks as shiny as the barrel wall itself.
Bottom line…Can you do a poor job of cone polishing? Sure. Can you tell by the “dark” appearance of the cone area? I don’t think so. The only way to tell for sure would be to section a barrel and look at it. But since the shot deformation in a plastic cup wad results from abrupt angle changes, not surface smoothness, if it doesn’t build up plastic and crud be thankful and find something else to worry about.