Literally the one difference between the "official", Hitler-approved definition of "assault rifle" and an AR-15 is that the AR-15 doesn't support selective fire†. As, like, literally any time spent in gun and military nerd forums or "I was in combat ask me anything" AMAs will quickly inform you, the importance of more-than-semiautomatic fire in actual military operations is highly contested.

Do I concede that the AR-15 is thus not an according-to-Hoyle assault rifle? Sure, but only Hoyle cares.

The reason this debate exists at all is because of "assault weapons bans". But the semantic argument is, of course, circular: it matters if a weapon is an "assault weapon" because we might ban assault weapons, and a weapon is an "assault weapon" if we might ban it. To reasonably discuss them as they pertain to public policy --- which is what this subthread is doing --- you have to engage on the merits. The appeal to definitions is a smokescreen.

† For the advanced Facebook version of this discussion, try making an argument about the legitimacy of binary triggers; is an AR-15 fitted with a binary trigger group an "assault rifle"? Well. Let me tell you something about fully automatic fire: it occurs only when you eject multiple rounds with a single depression of the trigger, and also, with a binary trigger, you have you depress the trigger a certain way to "select" rapid fire, which doesn't really count as "selection", so...