A M16A1 generally meets the definition of a Machine Gun.
By the way...An M2HB meets the general definition of select fire rifle.A machine gun is a fully automatic mounted or portable firearm, usually designed to fire rounds in quick succession from an ammunition belt or large-capacity magazine, typically at a rate of several hundred rounds per minute.
And the LAW makes no distinction between the two.A selective fire firearm has at least one semi–automatic and one automatic mode, which is activated by means of a selector which varies depending on the weapon's design.
machine gun is a technical term for any fully automatic firearm, and also for any component or part that will modify an existing firearm such that it functions as a fully automatic firearm.