The only fully rifled slug barrel worth having on a shotgun is one with a cantilevered scope mount so the scope stays permanently affixed to the barrel.
Otherwise, every time you swap barrels, or remove and remount the scope, your zero will change.
That's why I built a dedicated slug gun that I like to call my "Jersey Rifle", but I don't use it at all anymore, really.
Here's some more details about how I went about putting it together:
http://fishdogsguns.blogspot.com/2008/0 ... rifle.html