I used a green Scotch-Brite pad covered in JB bore paste and soaked in Kroil over a 20 gauge brass brush. I polished back and forth until the newly charged 18V battery died, I had to change the pad twice I think. After the battery died I changed to a 12 gauge brush and cotton patches and cleaned everything up with Hoppe's #9.
My barrel was rough, it looked like it was cut with a chisel.