Blackout

20-minutes in, Cain is firing a rifle in a hallway. The camera is behind him and over his right shoulder as he fires. It switches to slo-mo and you can clearly see the "spent" rounds being expended from the chamber are intact, rubber-tipped prop rounds meant to mimic the expended casings. Alternate Explanation: It appears that the shell casings the AR-15/M-16 series rifle is ejecting are actually from blank rounds. Instead of a rubber tip, the blank round has gunpowder in it, and they crimp the tip of the casing. This allows the rifle to fire, eject the round, and with the gas from the round, cause the next round to load, just as if it were a live round. This also explains why there is no muzzle flash since, for the process to work, they must plug the barrel.