I've got a system for you to try. I used it to bring back a couple of rifles from bad neglect, and they shoot tolerably well. With a smoothbore it should work even better.
Get yourself a correct sized cleaning jag, Track of the Wolf should have them. While you're ordering, order the next size smaller jag, you'll need it. Now, when I got my barrels, they had caked on powder and rust and crud, so I plugged the touch hole and soaked them overnight with Kroil... PB Blaster will work too, really any penetrating oil. (not WD-40)
When that was done, I took steel wool, fashioned it into a crude patch, put it on the next size smaller jag, and worked the bore with it. You could probably use the green scotch brite pads too. This part takes elbow grease and time. When my arm needed a break, I'd flush the bore again with a little bit of Kroil, and check it with a flash light, and maybe run a patch or two down to see how clean it's getting. I just let the gun tell me if it's done... if it looks like you're not making any progress, call it good and go make smoke!