Say ello to my leetle frein.
Hollyweird uses specially constructed weapons. They have a restriction inside the barrel so that gas pressure from the blanks is enough to cycle the action. This works like the blank firing adapters the military uses for training, but is internal rather than external. Additionally the blanks they use produce much more flash than regular loaded ammunition, or military type blanks. These are highly visible even in bright sunlight, let alone in the dark. More dramatic and photogenic don't ya know.