Spats correct me if I'm wrong.
In Arkansas a LEO may purchase and own an NFA weapon without going through the Feds. He may keep the weapon until he ceases being a LEO. The weapon must be passed to another LEO whose department authorizes the Officer to have one for Law enforcement use.
My training Officer had a mint condition Thompson with the round mag. (I'm not into Full auto so pardon any improper termoniology). When he retired he sold it to another officer.
The Tommy gun had an outstanding effect when Officers were severly outnumbered in a couple of tense situations.