From what I have heard, he had every reason to think that his weapon was safe to handle and use.
Several members of the crew walked off set earlier that day because safety protocols were not being strictly followed.
Hours before actor Alec Baldwin fatally shot a cinematographer on the New Mexico set of “Rust” with a prop gun, a half-dozen camera crew workers walked off the set to protest working conditions.
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Safety protocols standard in the industry, including gun inspections, were not strictly followed on the “Rust” set near Santa Fe, the sources said. They said at least one of the camera operators complained last weekend to a production manager about gun safety on the set.
Several members of the crew walked off set earlier that day because safety protocols were not being strictly followed.
https://www.latimes.com/entertainment-arts/business/story/2021-10-22/alec-baldwin-rust-camera-crew-walked-off-set