I recently was looking into the same thing. I have a mossberg 500 but I have a 28" barrel with screw in chokes. I wasnt sure how it would shoot so I stuck to my bow through gun season. If you purchase a new shot gun get on with both barrels. Most manufactures will have a setup with your typical shotgun barrel and then a slug barrel. I know that at walmart.com they sell a mossberg rifled barrel with a cantilever scope mount with scope. Its bore sighted and it is $150.00. I plan to buy one really soon.
good luck to you