Florida law states:
790.221 Possession of short-barreled rifle, short-barreled shotgun, or machine gun; penalty.--
(1) It is unlawful for any person to own or to have in his or her care, custody, possession, or control any short-barreled rifle, short-barreled shotgun, or machine gun which is, or may readily be made, operable; but this section shall not apply to antique firearms.
(2) A person who violates this section commits a felony of the second degree, punishable as provided in s. 775.082, s. 775.083, or s. 775.084.
(3) Firearms in violation hereof which are lawfully owned and possessed under provisions of federal law are excepted.
This thread started from a Florida incident, and if LEO did respond they could investigate and either arrest or impound the weapon.