will shooting buckshot from a rifled slug barrel damage the barrel?

Normal lead buckshot or copper plated buckshot, no. The barrel metal is much too hard to be worried about lead.The only problem is the rifling donuts the pattern. Meaning you can have big gaps right in the middle of what you are shooting at.

Also it may cause more lead fouling than normal, affecting the accuracy of the next slugs you fire until cleaned.

heavi-shot buckshot is another matter, which is superhard and contains tungsten. I would not fire this out of my rifled barrels.

It sure won't do it any good and spinning the shot collum will blow the pattern. Centrifical forces will cause the pattern into a doughnut shape, i.e. it will have a big spreading hole in it.

over time yes,and don't ever shoot steel out of it or it might only take once.