Rifled shotgun barrels are supposed to be designed only for sabot slugs. However, I have had fairly decent accuracy with mine when using Challengers, which are not sabots but I'm not totally sure if they're old-fashioned Forster style slugs either. They'll go into about 4 1/2 inches @ 100 yards. Of course, Winchester Partition Golds will print 2", sometimes better.

Rifled barrels are definitely not the way to go with shot loads. I'm assuming that the rifling imparts centrifugal force that scatters the shot. Whatever the explanation, you get HUGE patterns. Don't ask how I know. But shot loads are cheap, so you know you're gonna try it.
John