Admit it. You've never shot an AR-15, have you? Never done a mag dump with one?
You clearly don't know what you're talking about.
Yes. So many people are confessed experts. They don't know squat.
10 x 30 round magazines is 300 rounds. (I usually download to 28 rounds). 56 rounds can be fired in 60 seconds, including the mag exchange.
If the shooter was trying to simulate full-auto, good. More misses at his 400+ yard distance. Slower aimed fire would have been more effective.
But then what do I know? Facts about the weapon and shooting environment are hard to come by. Lighting and visibility at night is key. Ambient lighting and a 4-9x power optic would have made for devastating results.