This is factually incorrect. Don't take this as a judgement on you or your position, just that you should be approaching any side from a factually sound place.

"AR" in "AR-15" stands for ArmaLite Rifle. ArmaLite is the firearms manufacturer where Eugene Stoner was working when he designed the rifle.

Assault rifles, and most other weapons capable of automatic fire, are generally not intended to be used as "spray 'n pray" weapons. That is generally reserved for stationary machine guns (think the beach on D-Day).

Assault rifles generally are classified as weapons that fire an intermediary cartridge that are capable of select-fire. Meaning that they fire cartridges with size and energy in between centerfire pistol (ex. 9x19mm) and full-power rifle (ex. .30-06 Springfield AKA 7.62x73mm) and the operator may select between multiple modes of fire. Usually these are semi-automatic (one round per trigger pull), fully-automatic (continues to fire rounds while trigger is held down) and/or burst-fire (two to three rounds per trigger pull).

The use of fully-automatic fire on modern assault rifles is extremely limited, with standard issue military rifles in the US military having the fire mode completely absent until the recent switch from the M4 (semi-auto and burst-fire only) to the M4A1 (semi-auto and full-auto). Tactically, fully-automatic fire is usually limited in use to room clearing in close combat and for suppressing fire (keeping the enemy combatants from leaving cover) to allow the squad to break contact and retreat to safety. This is reflected in the types of units that have been consistently issued assault rifles capable of full-auto; generally special forces and reconnaissance units that may be deployed outside of range of friendly support.

Full-auto in an assault rifle is simply not very useful in modern military roles. A standard issue magazine holds 30 rounds. An M4A1 in full-auto fires about 800 rounds-per-minute. That means that it takes just a bit less than 2.5 seconds to empty an entire magazine, putting the soldier in the vulnerable position of needing to reload. In most situations, it's far better to employee a squad automatic weapon, which is generally fed by a belt with much higher capacity, allowing sustained suppressing fire to allow allies to maneuver.

All of that said, I do, personally, agree that civilians (including police forces) ought not to have military-like firearms primarily intended for shooting humans. But that is because I am mostly a pacifist. The ArmaLite Rifle (AR-15) is NOT a select-fire rifle but a semi-auto one. It can, however, be modified into one (illegally) and uses the same rounds and accessories. To me, that makes it "military-like" and should likely be heavily regulated (but won't be).

TL;DR - Whichever side you are arguing, do it with facts.