Does rifling have to be fully functional? Or just curving scrape marks down the barrel?
I remember seeing some sort of determining standard about that, but I'm unable to find a cite.
If I can't find a court case, or reference to a law, then it's probably up to the discretion of the ATF technical branch to determine if rifling is sufficient or "real" enough to avoid a smooth-bore AOW designation, assuming a "virgin" frame or receiver for the firearm in question. Assuming it's a breechloader that uses fixed ammunition.
So to that, I'd say, the more "real" the rifling is, the better. IIRC, some have even made the case that rifling need not even curve, but can be straight, as the first rifling was intended to provide clearance for bore fouling, and it was only happanstance that imparted spin was discovered to generate increased accuracy and repeat-ability.
Many have suggested that as a work around for the AOW category, to produce handguns that fire shotshells that could still transfer as Title I firearms without any NFA hassles, at least for shells smaller than .28ga (must be at or under .50, or it is now a Destructive Device, unless granted an exemption, such as .600 Nitro have etc.)
However, no manufacturer I'm aware of has tested the waters with that as of yet.
