Since you seem to know more about this than I do: could one deduce what was used from the audio recordings that I'm sure lots of people have made? I imagine some combination of rate of fire and pauses between bursts and other factors could tell if it was "real" full-auto fire.

I listened to the audio a handful of times. There is so much echo and microphone clipping that I couldn't determine.

I agreed with bump stock at first because it did seem to be erratic timing. It didn't seem like real auto. But after a few more listens, it's hard to tell, the echo gets really confusing.

I've been on ranges that I would swear a guy has a burst trigger pack but it's usually just echo or some effect of the supersonic crack and/or shooter's position under canopy.

The ATF did say there were no sear hole for a real auto trigger sear/group. An R/DIAS or Lightening link should have a fairly stable rate, but not always. There is typically a some tuning that goes into those per lower (shims, machining, glue, etc).

I'd absolutely believe trigger crank over bump stock. That was it ended up sounding like to me. Seemingly increasing ROF then stop.

IDK. If you set me down on a bipod, I bet I could fire as fast as that video with a competition trigger. It would sound similarly sporadic to what was in the video, for what that's worth! I guess that's the larger point, even if it was bump stocks, it didn't make much of a difference.

I'm skeptical of the things not mentioned in that event. But, I just know the photo released showed bipod rifles at the window and bump stocks don't work well with bipods. You need the gun to recoil into your finger which means it can't be supported / attached to ground at the handguard.