As someone who actually has worked on movie sets I feel like I can add a little something to this discussion.
Firstly, it's not unusual to have actual working weapons on set. Not the norm but it's a relatively common occurrence. However, the fact that there was live ammunition anywhere on that set goes against all standard procedure. It is not allowed. Ever.
We've all seen or heard about the lack of safety surrounding firearms on set. I've also heard some disturbing talk that cast or crew or both had been using the weapon in question for target practice near the set. Again, a clear deviation from industry practice.
Seems relevant also that the crew working on set were non union folks brought in to replace union workers who walked away over the brutal shooting schedules (no pun intended). 16 hour shooting days plus a two hour commute each way is not acceptable. Contributing factor? I'd argue as much.
As to the political aspect of the story, I find it noteworthy that Baldwin, a staunch democrat, was grinding those crews to the extent he was. So much for giving a shit about working men and women. Conversely, it's worth mentioning that Clint Eastwood productions are known for shooting in a much less taxing fashion. I'd heard it from several different craftspeople and worked with 2 folks who'd worked one of his shows who'd verify that. Anecdotal? Yes. Sue me