The lower is "the rifle" as far as the law is concerned, since it's what has the serial number.
I'm assuming you're saying M16 in terms of select fire, and AR15 in terms of semi-auto. If that's the case- without getting into SBR's and such- the lower is what has the trigger group, and is what will get you into trouble.
Unregistered machine gun= prison. Hope that helps.