i was reading in the ithaca catalog the other day that when they permanently screw the barrel into the receiver that it gives them better accuracy , now my question is beings it's a slug just how much of a gain in accuracy can you get ??

There's enough tolerance built into most shotguns so that every time you put it back together, the barrel doesn't necessarily point to the exact same spot. Most makers get around this by having a cantilever mount on the barrel. A friend of mine changed his point of impact 18" by cranking down too hard on the magazine cap of his BPS. It also allows you to mount a scope or sights on the receiver rather than on the barrel.