"The AD “did not know live rounds were in the prop gun” and the weapon was readied by an armorer beforehand, the affidavit stated according to the Times."
https://variety.com/2021/film/news/alec-baldwin-rust-assista...
https://variety.com/2021/film/news/alec-baldwin-rust-assista...
In any event, they have the shell (with possible fingerprints), and prints on the guy as well. 3 people are supposed to have touched the gun (armorer, ad, and actor), and only 1 person's prints should be on the shell, the armorer. No doubt forensics will be important in this investigation.