Why don't you bring this over to the shotgun forum, just copy and paste. I believe I know how to help, there are others there that are true slug shooting junkies.
I believe from what you said that you have 'smeared' the lands with lead and need to remove it. You can start with lead removal cloth and a button type 20 Ga cleaning jag. It will be a very boring process and DO NOT get any of the 'yellow stuff' on the finish of your barre, it will remove it. Back and forth 20 times, patch, new piece of removal cloth and try again, you will see the results.
Hope to see your name in on the shotgun side
Hope this helps, Good Luck and Be Safe