slide/bump fire; why else would he have 10 weapons? If he had a full auto belt fed weapon, he would be much more efficient with just one weapon. Also, if I had an actual full auto belt fed and was proficient in its use (which I am), I would not fire through an entire belt non-stop like he did, then pause to get next weapon, repeat. I would fire targeted burst, slight pause, targeted burst, etc.
He apparently had 8 different weapons, with mags/drums. He wasn’t firing controlled bursts because he was using it as an area weapon - firing from an elevated position into a crowd. For him, volume of fire was more important than accuracy. He had multiple weapons to prevent overheating/malfunctions from emptying mags/drums at the cyclic rate.