My current, subject-to-change, working theory is that gunmen were on the ground, (possibly dressed as ‘security,’ as in this video), spraying concert attendees with lighter round (9mm?), suppressed mini machine gun fire, while larger M60s were firing from above somewhere, possibly the nearby, small towers being a key nest — maybe not as much from Mandalay Bay?
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Scott Bennett clarifies on Facebook a likely error in this video’s title:

I don’t believe this was a pistol grip AK-47. Instead I think it could have been a pistol with a special automatic firing pin. Notice how the shooter can be seen loading a thin magazine into the weapon (which appears smaller than an AK-47). It also seems like it is loaded straight up, where an AK-47 has the magazine inserted with an insertion then a tilt backwards, rather than a straight up into the weapon insertion (like an UZI automatic or pistol).

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https://www.youtube.com/watch?v=suJ9_mcZ-iQ

100% Slow Motion Proof That Was A Pistol Stock AK47 Rifle Being Fired Into Crowd!