Drums aren’t illegal. Any competent machinist can modify. hussein sotero had 8 yrs to seed his minions throughout the military. Lots of stuff goes missing. They came from the local cartel connection. I’m sure there is regular route to LV from Mexico.
https://www.yahoo.com/news/first-look-guns-used-las-vegas-shooters-hotel-room-163623120.html
No drums here. Large capacity magazines.