"Umm, how does it not meet the bore size requirement? The bore size of a 12 ga is over the 1/2 inch limit that can bring it in to the DD area."
You're exactly right. My bad. I was tired and dehydrated from being out on the flightline in 110 degree heat all day when I posted that. Hot in the Mojave these days.
But in any event... the technical question still remains: Is the insert "by itself" a SBS at that point when found in compamy with it's host, or is the entire assembly a DD?