Why on earth are these stage guns not fitted with a sort of choke and receiver restriction so that real ammo can not be loaded or fired?
The stage ammo would have to be a smaller, oddball caliber.
My guess is that insurers, studios, and investors will develop new rules and require some form of independent monitoring on set. The fundamental problem of course is that creative types tend to be rule breakers, which makes it implausible to trust them to follow rules -- even about dangerous instrumentalities like guns.
As for the Rust shooting tragedy, there was a report to the effect that the young female armorer on set yielded to entreaties to permit use of guns and live ammo for weekend recreational shooting by one or crew members or actors. If true, that would expand the scope of potential civil and criminal liability.
Touch DNA on live rounds on set could prove dispositive in determining the source. Instead of being stalled, the criminal investigation may awaiting the results of DNA tests and forensic analysis of cell phone and text message data before filing charges.