Fox said that a bump stock allows you to fire 400-800 rounds per minute. It seems in Vegas that accuracy didn’t matter. He just wanted bullets spraying everywhere - knowing it would take people out.
The rate of fire of the M-16 is listed as from 750 to 900 rounds per minute, and you can sight the rifle while so doing. The rate of fire is dependent on the bullet weight (there are two bullets, 55 grain and 62 grain), and the heavier one is slower, thus reducing the rate of fire. At the higher rate, a 20 round mag goes away in a couple of seconds. Back when we were working to add a grenade launcher to the weapon, the military was busy putting a four round burst option on the fire control selector, because in Vietnam, troops were running out of ammo very quickly. Also, it was at that time that the 30-round mag was introduced.