If it's a lot of material I will use a router table, but if it is a small amount I stick some adhesive sandpaper on a wood dowel, copper pipe, or the actual barrel and run it back and forth in the stock. Start with #100 then work down to however smooth you want it. I usually lay the barrel on the stock and trace the barrel on the stock with a pen/pencil/sharpie so I have an idea of how much material to remove.