Hi Mad,
A fully auto AK with a 100 round double drum magazine unloads in about 10 to 15 seconds. After about three mags the barrel is almost glowing white hot. On youtube there are folks that demonstrate different products and I have seen an AK barrel literally collapse and fortunately it didn't explode in the guys face.

This shooter had to know a little about weapons, full auto fire and barrel failure. It only takes a couple of seconds to release and pop in a full mag. Most folks use a 10 round mag and also let the barrel cool down after blowing off a couple of mags. 

The tragedy is there were so many folks in that place that basically all he or they had to do was hit the tent. He would have had to purchase the ammo locally. I think in Nevada is an open carry state but there is still background check to purchase. I think a conservative estimate of shots fired probably 1500 to 2000 rounds.

It's an absolute insane tragedy.