Props guns are supposed to be made in a way that they will not even accept a real shell.
The people shot (one fatally, one injured) were not actors (crew member and director), why was the prop weapon even pointed at crew members?
https://www.cnbc.com/2021/10/22/alec-baldwin-fires-prop-gun-on-set-of-movie-killing-a-crew-member-and-injuring-director.html
"Hollywood adheres to Safety Bulletins, written and distributed by the Industry-Wide Labor-Management Safety Committee, for standards on weapon and prop safety, as well as other rules.
In the first bulletin addressing firearms and the use of blank ammunition, the document notes prominently: "TREAT ALL FIREARMS AS THOUGH THEY ARE LOADED." It goes on to say live ammunition is to never be used nor brought onto a set.
However, these are recommendations and not binding law."
https://www.npr.org/2021/10/22/1048295916/props-gun-death-injuries-rust-movie-set-rare