Is the gun you are looking at a 500 or 400? Both have many common parts. Barrels are the most obvious. The criteria for interchangeability is that they are the same gauge, chamber length (99% are 3" chambered but some very early ones were 2 3/4" only, barrels still interchange but limit to 2 3/4" shells) Persuader barrels and other 500 models with long magazine tubes will not fit standard 500/400 models due to the location of the barrel support ring. Fore ends and fore end iron are a whole different issue. I don't have the time to type out all the possible scenarios when it comes to 400/500 series shotguns and fore end details.
Darryl