The AR-15 and like weapons fire the .223 caliber round, sometimes referred to as the NATO round. It is erroneously described as "powerful", but is much smaller and lighter than the 30-06 round our fathers fired from their Springfield rifles in WWII. The doctrine behind the NATO round is that wounding a soldier is more effective than killing him out right. You kill one soldier, you've eliminated one combatant. You wound the same soldier instead, and roughly ten to twelve men will be tied up caring for his wounds and transporting him for medical attention.
While any bullet from the lowly .22 short or .25 can be lethal with the proper placement, the .223 was designed to be less lethal than previous weapons of war, to tie up the enemy's resources and to possibly allow some of its fighting men to live, to return home after the fighting stops.
Consider that 103 people were shot in Orlando and over half of them, so far, survived the shooting. Knee jerk reactions to ban the AR style of rifle may have the unintended consequence of putting even more lethal firepower into the hands of some future evil assassin.
A Response to Yet Another Knee Jerk Reaction to the Orlando Shooting