According to Curtis H.A. Bartlett, Acting Chief, Firearms Technology Branch, in a letter dated, JUN 19 2000:
"...As defined in Section 921(a)(30)(D), of United States Code, the term "semiautomatic assault weapon" means a semiautomatic shotgun that has at least 2 of the following features:
(i) a folding or telescoping stock;
(ii) a pistol grip that protrudes conspicuously beneath the
action of the weapon;
(iii) a fixed magazine capacity in excess of 5 rounds; and
(iv) an ability to accept a detachable magazine.
Section 922(v)(1), of Title 18 U.S.C., provides that it shall be unlawful for any person to manufacture, transfer of posses a semiautomatic assault weapon. However, paragraph (v)(3)(D) provides that the prohibition in paragraph (1) shall not apply to any semiautomatic shotgun that cannot hold more that 5 rounds of ammunition in a fixed or detachable magazine.
If the described shotgun cannot hold more than 5 rounds of ammunition in its fixed magazine, it is excluded from the prohibition in section 922(v)(1)."
From the MAN.