A ban on "assault rifles," if attempted, has to address speed of firing, magazine capacity, and ease of conversion, because the AR-15 semi-automatic isn't an "assault rifle" in military terms. That being said, what's available now at your friendly neighborhood gun shop, or show, or on the Internet is more than adequate for killing a "mass" of people very quickly (the weapon can fire as fast as you can pull the trigger, with skill, 3 - 5 rounds per second, 180 - 300 rounds per minute). These sort of firearms should be banned.
RESPONSE
I would like to make a technical comment as a PhD Mechanical Engineer. Recoil reload is the simplest mechanism in small arms (nearly all semiauto pistols and many submachine guns use recoil reload) and very fast and efficient. For larger cartridges, gas reload is the mechanism. This includes most high powered assault rifles such as M-16, AK-47, etc. AR-15 is an M-16 variant without a selector switch.
These mechanisms are all either incredibly simple or very simple. The only practical way to regulate is on magazine capacity and how easy it is to change a magazine. Faster reload mechanisms are mechanically forced such as on modern Gatling guns for example (up to 4000 rounds per minute in cannons on aircraft and close in weapon systems to shoot down missiles about to strike a naval vessel). If you make it illegal to manufacture quick change box magazines of any size (obviously include belt or chain magazines as well), the problem is partially solved. The box, belt and chain, reload devices were invented for military purposes and frankly have no other purpose (it's not necessary for any use that doesn't require high volume "suppressing fire"). Regulation of magazines is an Achilles heel of the "assualt rifle" market.
COMMENT
I agree that both magazine capacity and ease of changing the magazine would have to be addressed. Otherwise, shooters intent on murdering people simply bring extra magazines. However, it's not clear to me how to change the process of loading a second (or third or fourth) magazine to make it hard enough to interdict the shooter.