I'm not quite so sure about this. I asked the thoughts of others on this elsewhere, and didn't get much of a response. The language in the GCA that restricts imported firearms is pretty much identical to the language that allows a shotgun with a bore over .5" to be a title I firearm - otherwise everything over .5" is a DD. Both require that it be "particularly suitable for sporting purposes"....
So... it would seem to me that if a shotgun of a particular configuration is not allowed for civilian import because it is not "sporting", then it could be said to become a DD by default, regardless of parts count, since the same criteria is used for both.
And yes, I know that to date, the only SG's determined to be DD's were declared so by name. However, that does not have to be the case (and in fact, the wording of the law seems to indicate the opposite should occur), and the expiration of the AWB puts us in a place with this that we've never been before.
Thoughts?
Rocko