There’s an entire industry surrounding the production of (often incredibly) realistic not-firing (and blank-firing) prop guns. the only time you’d need a real one, firing real bullets is if you were doing some extremely-close up shots or recording sound. Even then, you’d only need the real bullets for sound effects or close ups of actually firing. The only thing you’d really need CGI for is the muzzle flash. which is so short and so slow most people would barely even notice if it was merely ‘realistic’.
All of which, it should be said, could have been shot with no one down range of the weapon at any time, and in any case, there was zero reason to need a functional firearm at the time of the shooting. They were not actually filming. They were setting up the cameras and checking for things like glare and reflections and various other angles. All of that could have done with a non-firing prop with no danger to anyone at all.