LIke already posted, shooting shot through a rifled slug barrel isnt going to hurt it other than getting alot of plastic fouling from the the modern wads used today. The rifling is going to act like a spreader choke simular to a Briley choke I use in some sporting events. It is going to impart a twist/spin on the wad thus spreading the shot as soon as it exits the barrel. You should be able to cover the average man sized target at 5 yards with it.

The same can be said with shooting the .410 shotshell in the rifled barrel of the Taurus Judge, it works well at close ranges only.