The bit I heard didn’t sound like 7.62 to me either. He apparently had a bunch of weapons up there, but I was almost certain one clip I heard was full auto 5.56. First thing that crossed my mind was M249 SAW in terms of a similar rate of fire.
I can't listen to any clips right now to know, but is it at all possible the audio includes echos of the rifle reports that make it seem like more are being fired than really are?
Just throwing that out there. I'm working from the KISS principle and also assuming that most criminals are stupid, so I'd guess this particular reject just grabbed a bunch of guns and magazines and did what he did, nothing special, just your typical, evil sub-human.
On that principle I'd assume he booked a room only a few days in advance, probably got a cancellation that he was able to pick up, or got a suite at an extra cost (those are sometimes available with less advance notice since they'll hold a few for people willing to pay for the privilege). No real advance planning besides checking to see if a room was available, preferably facing that direction (if not he probably would have picked some other target(s) of opportunity).
All my speculation is just that, but based on that premise that criminals are generally idiots.