Um... rifled barrels are really only for slugs. You technically can shoot shot through them, but I would say that you couldn't shoot at anything say more than 20 yards away and have your shot be worth a damn. If you are trying to shoot an animal though, shooting a shot shell through your barrel is just going to piss them off.
The main problem with shooting shot out of a rifled barrel is that the plastic from the shot wads will clog up the rifling in the barrel. This can be a real bear to get clean. It doesn't actually hurt anything as far as the shot is concerned. You might even get decent patterns with it. It's just that if the rifling is clogged up, you won't get very good accuracy when shooting slugs. Rifled barrels deliver best accuracy with sabot type slugs. The rifled "Foster" type slugs give better accuracy from smoothbore barrels.
Let me qualify my above comments by saying that I haven't tried shooting shot from a rifled barrel, but that this is my understanding of what would happen. Perhaps someone who has actually tried it will tell us their experience.
Yeah, Guest, we're taking the wrong tack with this answer--I started it too, so it's my fault. Yes, it's SAFE to shoot shot out of your rifled barrel, it's just dumb. The thing to do is visit Mossberg's web site, where they will sell you (direct order I think, or at worst pick it up at a distributor) a nice new extra smoothbore barrel. This will solve your problem. What are you going to be hunting or shooting at? Tell us and we'll give you even more great advice.
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