An interesting thing about the writeups in this node is that most of them deal with the question of "what is an assault rifle?" rather than the stated question of "what is an assault weapon?". Legally, according to the 1994 "Violent Crime Control and Law Enforcement" (complete text with commentary found at http://www.recguns.com/IIG4.html):
A semiautomatic rifle is an "assault weapon" if it can accept a detachable magazine and has two or more of the following:
A folding or telescoping stock
A pistol grip
A bayonet mount
A flash suppressor, or threads to attach one
A grenade launcher.
A semiautomatic shotgun is an "assault weapon" if it has two or more of the following:
A folding or telescoping stock
A pistol grip
A magazine capacity of over 5 rounds
A detachable magazine.
A semiautomatic pistol is an "assault weapon" if it can accept a detachable magazine and has two or more of the following:
A magazine outside of the grip
A threaded barrel to accept a flash suppressor, silencer, etc.
A barrel shroud
A weight of 50 oz or more, unloaded
"A semiautomatic version of an automatic firearm."
As you can see, many of the qualites of an "assault weapon" are purely cosmetic. Also note that, unless it is specifically mentioned by the bill (again at http://www.recguns.com/IIG4.html), a fully automatic weapon is not an assault weapon!
The bill also bans what it calls "large capacity ammunition feeding devices" (LCAFDs). An LCAFD is any magazine that can accept more than 10 rounds of ammunition (except .22 caliber ammo).
It should be noted that assalt weapons are used in only one-fifth of one percent (.2%) of all violent crimes and only about one percent (1%) of gun crimes.