ADHESIVE SANDPAPER (fine grit)
Cut a small 1inch X 2inch piece and stick it to a dummy round. Thumb it in and out, dont press it all the way in as to let it go past the shell stop.
Doing this will create a polished indention on the shell stop lip.
Stop sanding when your satiated.? and dont sand all the way through it...lol!
Smooth as butter when your done!