use a small wooden dowel with a slot cut in one end about half an inch down the middle. wrap a small piece of 400 grit paper around it. touch it to the bottom of the chamber while the barrel is spinning in a lathe, pushing it forward slowly until you feel it touch the shoulder and then come back with it, repeat until the chamber is very shiny inside, you can also use some kerosene on the paper to help speed it up. thats how i have done it.