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.
it may be a myth.