The bigger failure was the both the production company(s) not rigorously enforcing redundant safety standards (cast+crew double checking weaponry after the armorer), and crew not reporting unsafe conduct to a regulatory body or insurance company, if there was a mechanism with real consequences to do so. Frankly, the film industry should require firearm competence certification for anyone handling a gun or realistic prop. All the above are much more important than the 2nd rule by itself.