This was possibly intentional sabotage from one of the crew, or several crew members. It seems this was a troubled shoot and many of the crew were pissed at the higher ups for one reason or another, and film crews are notorious for doing stuff like this when they feel they've been mistreated. It's very possible a crew member intentionally put live rounds in these weapons with the hope something bad would happen, which it did, which would make this a crime, maybe even homicide.

I've been on sets and worked with crew myself in the past and honestly, it wouldn't surprise me. Crew work isn't glamorous, it's essentially glorified blue collar work; long hours, low pay (unless you're union), and often questionable treatment. You don't have to be educated or even a decent human being to get a job on a production; sometimes, it helps to be neither. You just need to know the right people. And many who work these jobs are less than savory types: people with criminal records, or who come from "rough" backgrounds.

It's always struck me as odd that so many wealthy and famous people working productions often depend on the labor of individuals who could easily have been released from prison for violent offenses only a few weeks prior to the job, or who otherwise come from backgrounds you'd likely never encounter in a more "corporate" type of environment. Well, this is what happens when the two mingle. Frankly, I'm surprised it doesn't happen more often. It likely does, we just never hear about it unless it involves a death.