According to this page (damn it, I can't post any links yet, I'll obfuscate it):
w w w .njguns.com/define.htm
.. the relevant part says:
w. "Assault firearm" means:
[snip...]
(3) A semi-automatic shotgun with either a magazine capacity exceeding six rounds, a pistol grip, or a folding stock.
NJ is the worst...