Croatoan
They/Them A-10 Warthog
I doubt he had true fully automatic weapons. He likely had a gat crank mod on an AR-15. The rate of fire in some of the videos was irregular. Seems very likely to be a semi-auto rifle that was modded.
Now some of these mods are legal now and may be banned. But a gat crank could be a DIY job if you have the skills and tools.
Another possibility is a bump stock mod.
I think you are on to something. These mods should be illegal with the same felony status of having a unregistered fully automatic gun. I also think there should be required "gun insurance" that owners pay into like we require car insurance.