Yikes!
AR-15 stands for ARmalite model 15. Not Armalite Rifle, not Assault Rifle, not Automatic Rifles.
The original AR15s were select-fire rifles. They didn't become M16s until these select-fire AR15s were given a US Military designation.
An assault rifle is specifically a select-fire rifle (NOT just a full-auto) that fires a mid-power rifle round, and uses high-capacity detachable magazines.
The term Assault Rifle has a specific military meaning (see above), which is very different from the catch-all misnomer "Assault Weapon", a term invented by HCI that is constantly being "redefined" for political reasons.
And LEOs RARELY have full-auto-capable rifles, except SWAT-type units. Most LEO ARs are commercial Colt, Bushmaster, or Armalite models, and while they may be in "pre-ban" or "AW" configuration, they are rarely select-fire. And the military surplus M16A1s that are given to police departments usually have their select-fire components removed, or have a block added to prevent the selector from going to the AUTO position (like most Reserve and Guard units).
Granted, today, most people use the term AR15 to denote a semi-auto-only rifle, and M16 to denote a select-fire rifle.
-Troy