If the bore needs smoothed up from manufacturing burrs/sharp edges, or light corrosion, and you don’t want to try shooting smooth, you could try Scotch-Brite. Here is a @duelist1954 video showing the process he used on a difficult to load GPR, a rifled bore, not a smoothbore, but the process would be the same.
I have used a similar method with Scotch-Brite for some time, but Mike explains his very well. I started doing it after speaking to Don Getz (Getz Barrel) years ago before he passed, and he recommended using the green (600 grit) Scotch-Brite for smoothing up barrels that were cutting patches. Said it wouldn’t hurt the barrel.
If you are attempting to correct issues brought on by neglect or poor cleaning practices, you may need to consider lapping or refreshing the bore, depending on how ‘bad’ the condition of the bore is.
Here’s how I ended up polishing out a slightly rough spot in my smoothbore about 10” from the muzzle: I screwed a .50 caliber brass jag onto a jointed caliber .50 machine gun cleaning rod using two sections. Using Scotch Bright household cleaning pads from the grocery store at about 600 grit I cut a strip and made a patch which fit rather snug in the bore. Then I smeared 320 grit polishing compound on that, chuckled it up in an electric hand drill and spun it on low speed. I polished out the rough spot, looked at it with a borescope
and patched it clean. Thanks for starting me in the right direction.