Well, the ATF regarded the Striker-12 and USAS-12 as unregistered 'destructive devices', but there was no special license to keep the one you already owned.
If the state you lived in had language in it's civil code prohibiting NFA-registered destructive devices, then you were screwed but there are few states that have such laws.
Only dealers, importers, and manufacturers of NFA-restricted firearms need any sort of 'license' and/or 'special operational tax' (SOT) classification.
Next target against Title II code is removing 'Sporting Purposes' language from the 1968 GCA in light of the USSC's 'Heller' ruling. The founders never intended a restriction on private citizens inside or outside the Militia Clause under the notion of what's 'sporting' and what's not.
Actually, they required you to register them.
http://www.atf.gov/regulations-rulings/rulings/atf-rulings/atf-ruling-2001-1.html