I'm sorry, Ed, but I have no information about that. The gunsmith told me that my shotgun has wide barrel wall and the oversized to 24 bore (overvored). In the barrel table there's .579: 24 bore, not 28.
That surprise me, so I'm looking for info to resolve it, but there is poorly info about Wiggan & Elliott makers.

Here you can see the Wiggan overbored barrel next to a normal SxS 28 ga.

[Linked Image from i.ibb.co]

So hope you can help me.

28 ga, hammerguns and all shotguns and rifles made by hands.
Waidmannsheil 🌿📯