The good Dr. was kind enough to point out in one post that there were no lives rounds allowed on the set. I'm sure she'll recall it. It was part of something she copied and pasted. It's back there around 30 posts or so.
HUH
Alec Baldwin's Gun in 'Rust' Shooting Was Used for Target Practice by Crew Members: Report
Alec Baldwin's Gun in Rust Shooting Used for Target Practice: Report (people.com)
Who brought the bullets that weren't supposed to be there?
I thought there were rules against that. No live rounds on set/property.
Did Baldwin bring them?
Did the Armorer?
The bullet gremlins, mayhap?