While the NRA may provide the definition of assault rifles as strictly those capable of of automatic fire, it's merely a matter of semantics. For the purposes of legislation, that definition is not accurate according to federal and state law definitions, in which the term assault weapon refers primarily to semi-automatic rifles, pistols, and shotguns that are able to accept detachable magazines and possess one or more other features.

Basically, in your world, an AR15 isn't an assault weapon. In the real world, it is.