A friend regularly shoots foster style slugs through a hastings barreled 11-87 after 10-12 rounds you cant see the rifling at all. So basicly he has a smooth bore. Soft lead or copper coated buck will as stated probably damage the barrel.It is very likely going to open up your pattern considerably. The shot column will be spun by the riflings and this will cause the pattern to open up sometimes even forming a doughnut shaped pattern. At HD ranges probably not an issue but at longer ranges it will definitely be noticeable
Another possible problem will be leading in the barrel. Depending on the manufacture some buckshot pellets might contact the barrel. Most quality buckshot is completely inside the wad and never touches the barrel but this is not always the case. The lead buckshot pellets if the do contact the barrel will likely cause some leading. Not a big issue, it will just require more elbow grease to clean.
ETA: is will not damage the rifling or barrel. Remember the barrel is steel, the buckshot is lead. You shoot lead bullets in steel barrel and never cause damage. It will possible lead up the rifling if the pellets contact the barrel and be a pain to clean but will not damage the barrel.
+1. I shoot rifled slugs out of a smooth-bore 870 with an 18" barrel. With Brenneke Black Magic and Federal Vital-Shok rifled slugs, my 870 will shoot 3" groups out to 50 yards and just under 4" out to 75. I wouldn't trust them past that, but that accuracy is good enough for deer/brush-gun use out that far.Those are fairly accurate in my 18.5" Mossberg,
out to about 50 yards.