Not all shot loads are in shot cups, wrapped and buffered. Evenb with the shot wads making a layer between barrel and shot, a rifled barrel or a soft barrel will be damaged bu hard steel shot.Any fouling should be from the plastic wads, as the lead or steel never touches the barrel.
Thats a good point about some barrels being rifled and some that arent. I have both. You can even buy a rifled choke for a barrel that isnt rifled if you want more accuracy for a slug.
I have a Mossberg 500 with a smooth bore that is accurate to 100 yards, which I know from personal experience. No exaggeration. This is with the cheap Remington slugs.Some "slug barrels" are just a regular short barrel IC choke with sights. Some are partially rifled and some are rifled full length. Some have shallow rifling, some are real rifled barrels intended for sabot slugs only.
Some states only allow shotguns for hunting deer and the modern slug-gun is accurate to over 100 yards, maybe even 200 if you know the drop.
Shot won't hurt the barrel, except steel shot will probably damage the rifling, lead will just foul it and it will be hard to clean.
An "old tech slug barrel" is great for quail hunting., IC 20"...