I ran up on this a while back.

http://www.assaultweapon.info/

I read a article that talked about the origins of this to but may have to dig a bit to find it. Basically, it's a made up term to make a AR seem worse. Thing is, a rifle with a traditional wood stock shoots the same round as a AR. If anything, most AR rifles have somewhat shorter barrels. Either rifle can be just as deadly as the other.