Mythbusters also shot into pools, and a few custom built tanks, and (later) learned that the bullet / explosion proof shields were actually NOT properly rated. But they did consult and presumably follow the advice of firearms and explosives experts on set.
Similar edutainment shows (I’m thinking R. Lee Ermey’s ‘Mail Call’ and the like) also show the host using firearms in a (generally, but not always) safe manner on-screen, with at least the appearance of being fired live. But in these cases, the firearms are expected and (hopefully) treated as live weapons, without the maybe-is-maybe-isn’t of many Hollywood shoots.
Presumably part of the problem is that a ‘real’ looking replica that can accept blanks but not not otherwise live rounds would be much more expensive than a live firearms. Economies of scale and all. Not that this wouldn’t be bad legislation to adopt, but it would be state level, and might again push productions to areas with less regulation, which was reported as an issue for the Rust shoot.
I am not trying to dismiss efforts to pass industry regulation or laws designed to protect against a repeat of the Rust or Crow tragedies, and it was correctly cited upthread that the industry standards if applied correctly would have prevented the most recent tragedy, but finding a one-size-fits all solution might be difficult.
If I were given power to make the regulation, I would probably require a custom pistol grip and action that could only fire blanks, that could be socketed into a wide variety of polymer ‘shells’ that replicate the appearance of a range of firearms.
You’d have a blank-only-firing ‘weapon’, that wouldn’t look anything like a real gun except in general outlines, and a realistic, but inert frame for appearance. Alternately, few outside gun and/or history enthusiasts can easily tell the difference at sight between various types of polymer semi-autos and revolvers, allowing for the creation of a class of realistic replicas that are blank-only firing and otherwise ‘close enough to pass’ visually.
But there is always room for human error. As an example specific to an OP (although one I would not agree with) - it would be LEGITIMATE in various legal municipalities to carry a personal concealed weapon with live ammo on the set. And some CCW people are . . . lax about keeping possession of weapons under their direct control. Which could result in a live weapon, with live ammo being left in a position where another incautious person could use it assuming it was a prop.
It would require levels of negligence that would be criminal, but it would be both legit reason to have live weapons and ammo on the set and LEGAL - just not smart.