It is difficult to tell the difference between an M-16 / M4 and an AR-15 unless you can see the selector switch/safety up close. My guess is that Government employees/LEO's would have the mil-spec versions.
The Honorable Jeff Duncan is a civilian and is probably more familiar with the firearms which are available at his local gun shop, the AR-15. Truthfully, they all qualify as AR-15 variants, some with select fire, others without.
Slide fire on an AR 15 trumps a three round burst on M16.