NZ shooter:
Dog & Bone = Telephone?
Did he call his therapist/handler prior to the act?
5.56x45mm is a high pressure round, and relatively speaking, has quite a concussion especially in a confined space. I am not an AV tech kinda guy, but there are plenty of firearms videos online where the camera mic clearly has some type of limiter/filter, which tries to level the recorded volume automatically. All the ambient sound goes quiet and then picks back up when the firing stops. This could be a reason.
I am not saying this is not a FF, but the video feels legit. I think it has something to do with the fixed focus giving people problems. You can see shell casings on the ground if you look careful. I don't think you would be able to read 5.56 entrance holes in a wall on that vid.
If the vid was faked, it was an excellent job. To a person with firearms experience, it looked like legitimate discharge. Use your own objective reasoning, and your mileage may vary. I quit watching on the 2nd go in. Too vile and revolting.