Fletch,

They will not hurt the rifleing, but will start to fill it with lead which will decrease accuracy. I've seen some rifled barrels shoot winchester super-X rifled slugs quite well. I have also seen a remington 870 rifled barrel throw slugs 6" apart at 50 yards using winchester super-X rifled slugs. It will not ruin the barrel, but defeats the purpose of having a rifled barrel.

KJ