He was not on a 'no fly' list. Or found by a court to be mentally ill. And he passed the background check according to the FBI.
The almost ten year 'assault weapons ban' which ended in 1998 'had no statistical impact' on murders according to the FBI and DOJ. 
And first you have to define what an 'assault weapon' is. Just being a black rifle isn't it!
With 22,000 firearms aiming at one window? Each person with a single California-sized ten-round magazine? 220,000 bullets? 
.
.




Yeah, I think that would have done it!

If just one relative of each of the 550 victims shot back with a ten-round magazine, that would be 5,500 bullets impacting the shooter's area.