Really, these guidelines would have prevented the use of real bullets allegedly mixed in by the prop supplier?
Yes, really. Among other things the guidelines prohibit any real live ammunition on the set. There should be an armorer on-set whose sole responsibility is checking guns in/out and ensuring they are unloaded, or properly loaded with blanks only when absolutely necessary. Only people who have been trained in the safety guidelines should ever handle them. Each person who handles a gun, right down to the actors, should also inspect it, and treat it as loaded even when it isn’t.
You cannot reasonably argue that it’s safer that an actor should read and learn what 40 years of experience and numerous accidents have taught an expert
I never said they did. It’s the responsibility of the producer(s) to ensure all regulations are followed. So they should have made sure the armorer did. It’s the job of the armorer to know the OSHA and other regulations involving firearms on-set, and adhering to them. The armorer should be instructing both the relevant cast & crew on established safety procedures. That should include how to safely check if a gun appears to be loaded, and if not 10000% sure, to check back with the armorer. Not with a random crew person but the person directly responsible for their safe use.