BBM
Maybe if the set was running low on dummies and was rushed to film so no time to go buy more?
It was a squib load that killed Brandon Lee and where the dummy bullet came from has no bearing on his death, but in that production, the crew made their own dummy bullets.
If this was the case on the set of Rust, and I have no evidence of that, I'm just stating it as a hypothetical, is it another way a live round could be mixed up with dummy rounds if people are rushed, inexperienced, and disorganized?
No Charges Filed in Actor's Death During Filming (Published 1993)
Dummy cartridges are supposed to be rendered harmless by removing the gunpowder and triggering the firing pin. Professionally made dummies can be bought,
but the ones used on the set of "The Crow" were made by crew members.
In the close-up shot, the triggering device in one dummy bullet apparently was improperly fired. The cartridge popped during filming and lodged the bullet in the tip of the gun barrel until it was dislodged when the blank was fired. The gun was not inspected between the filming of that scene and the scene shot on March 31.