The ArmaLite AR-15 was a select-fire, air-cooled, gas-operated, magazine-fed assault rifle manufactured in the United States between 1959 and 1964. Designed by American gun manufacturer ArmaLite in 1956, it was based on its AR-10 rifle. The ArmaLite AR-15 was designed to be a lightweight assault rifle and to fire a new high-velocity, lightweight, small-caliber cartridge to allow the infantrymen to carry more ammunition.[4] The rifle was not available for civilian use.

In 1959, ArmaLite sold its rights to the AR-10 and AR-15 to Colt due to financial difficulties.[5] After modifications (most notably, the charging handle was re-located from under the carrying handle like AR-10 to the rear of the receiver),[6] Colt rebranded it the Colt ArmaLite AR-15. Colt marketed the redesigned rifle to various military services around the world and it was subsequently adopted by the United States military as the M16 Rifle, which went into production in March 1964.[4][7] Colt continued to use the AR-15 trademark for its line of semi-automatic-only rifles marketed to civilian and law-enforcement customers, known as Colt AR-15. The Armalite AR-15 is the parent of a variety of Colt AR-15 & M16 rifle variants.

It was also redesigned for bullet rotation so that it is much more ACCURATE.

The shooter in Vegas was not on any medications to my knowledge. Mental illness is much more complex to treat/handle. Removing these guns is a much easier fix. Definitely need better gun laws. Anyone can buy a gun at a gun show with NO checks. So wrong. Bad people will always find a way around it.
JMO