Good question. I want to know the answer too.
I have a Remmington 'leven hundred with a smooth barrel, but I'm in the market for an 870 with a rifled barrel.
I've been doing some looking around and shooting ad couple of borrowed 870's.
I've heard that a rifled barrel will cause some kind of a donut pattern in the shot, but I have no personal experience actually seeing it done.
someone will know, though, I'm sure. If not, we'll get Frost to shoot a couple of paper targets and post them.
If your not shooting it on a regular basis then it won't be a problem, However it will wear the rifling down over time with continued use. My Dad had a .22 he use to shoot rat shot out of to keep the mice and rats down in his chicken house, Needless to say the barrel was almost smooth.
Although I would not fire ANY steel shot through it at all.
Conventional slugs may or may not work better and may or may not lead.
I would have no hesitation to shoot lead shot in a defensive role although according to the guys at "Box of Truth" it will shoot in a donut pattern.
According to them it opens way up and they came to the conclusion that past about 12 yards you will miss with more shot than you will hit with.