Hi from a DD dealer.
DDs have no barrel length rules. It comes down entirely to bore size. If it's a bore over 0.5" and it's not a "sporting shotgun" or a muzzle loader, it's a DD.
(Think about it- do you pay two taxes for one M203 because it has a 9" or 12" barrel?)
Suppressors, unless permanently mounted to an air gun, are another NFA item. It
used to be that permanently mounted suppressors were exempt. For example, a MP5SD with a welded-on can was a "one tax gun." ATF doesn't allow this any more but grandfathered the existing ones. Most are getting removed, as suppressor technology has changed and MG buyers aren't worried about saving $200 on a $25k+ firearm any more.
So, DD + Suppressor, whether or not permanently affixed and barrel length being meaningless, is two $200 taxes.
Mike