So this pistol doesn’t count as a “sawed off shotgun” because it has a rifled barrel? I don’t know how the regulations work here.
There is an explicit clause in NFA'34 that states that the term "short-barreled shotgun" does not include pistols with rifled barrels. Absent such language, the BATF might very well decide that any pistol for which shot shells are available was a "short-barreled shotgun".
The reason I was thinking of 28ga as being interesting for self-defense would be that there is language defining any firearm with a bore diameter greater than 0.5" as a 'destructive device' unless either the Secretary of the Treasury finds that it is suitable for hunting or sporting purposes, or it falls under some other exemptions (flare guns, antique/replica muzzle-loading cannons, etc.) A 28ga shotgun is nominally I think 0.505", but I wouldn't think there should be any problem with a fixed choke that constricted it to 0.495". Indeed, many chokes would probably constrict more than that.
BTW, I'm curious: if one wanted a mostly-cylindrical 0.495" barrel, I wonder how long a forcing code would be required for safe use? It might be possible to put a rifled barrel on a 28ga revolver without it having to be too outrageously long.