The 13.5 is referring to the choke, which is .531 and is marked that way on the bbl flats. That would make it full choke for a 28ga.
but the bore also being .531 doesn't allow me any material to remove. A 28 ga shell fits in tightly in the chamber and the owner tells me they are
hard to remove after firing. But this could be because he is using 2 3/4 " shells in a 2.5" chamber.