I believe they may have been using Simunitions. It would produce the illusion of a functioning weapon.
I've been hit with that stuff before. It will make you wince, but it won't kill you. SWAT and SOCOM love it and train with it constantly. However, you have to be a govt entity to get your hands on it.
The tell-tale would be the expended brass left behind by the shooter.
Can't get a good look at that brass because of the blurry video. I still think they were using Simunitions. It's the only explanation for no blood spatter other than a Hollywood blank adapter in the barrel. Blank brass is easy to spot because it is crimped at the business end to hold the powder in without a projectile.
No gas, no ejection etc could be a normal stoppage due to the bolt not fully locking. Pretty common in AR's that aren't cleaned properly.