They shouldn't even have a real gun on the set. Hollywood folks are starting to realize that. The call now is for all of these things to be replicas and they will add the flash/bang in post production. I doubt most of these films use the original sound track anyway. They shoot the action and "Foley" in the sound later most of the time. With CGI the muzzle flash is trivial to do and they can even make it more spectacul;ar than real life. That does seem to be the goal anyway. This is a movie, not real life.