The term "assault rifle" defines a specific type of military weapon, regardless of what someone who does not understand the term erroneously considers it to be.  All assault rifles are capable of "selective fire," that is, firing "full auto" in the manner of a machine gun.  Machine gun vs. semi-automatic is a rather important distinction and certainly not just a technicality.  The author seems to understand this because in the text he refers to the gun as "an AR-15-type rifle," not an assault rifle, M-16 or machine gun.  He just couldn't bring himself to get the headline right.