Those could actually be AR-10 type rifles. They look like an AR-15 but is just a bit bigger. What I've seen on some of those pictures is bumpfire stocks on a couple of those rifles. Done right, bumpfire can be pretty regular and fast, but like FA, it's not very accurate. Still, when it takes a few minutes for LE to get to the shooter and plenty of ammo and a crowd of around 22k, hitting around 600 people shouldn't be all that difficult even from 500 yards (or less) away.
Also, for AR variants, the only NFA component of a FA gun is the autosear. That's considered the machinegun. All the other parts can be from a machinegun but as long as the receiver will not accept the autosear, the resulting rifle is NOT a machinegun.