No, it's not. Assault rifles have burst and full-auto settings, by definition. The AR-15 does not, only semi-auto, and does not have burst or full-auto settings. Ergo, it is not an assault rifle. It is a semi-automatic rifle with a box magazine. Just like the Barrett M90, the Dragunov SVD, the Bushmaster M17, etc.
Words have meaning. Use them properly.