From your link:
That Thursday, the crew began preparing for the scene before Baldwin had returned from his lunch break. Gutierrez Reed entered the church with the firearms, performing a safety check with the Colt .45 in front of Halls. He thought he saw three rounds inside the gun, but he did not check them before taking the weapon in his hand.
He told investigators that “he should have checked all of them, but didn’t, and couldn’t recall if she spun the drum,” according to the affidavit.
The armorer left the church.
No stand-in performers were on site, so the first assistant director ran through Baldwin’s blocking himself, pulling the gun three times. Russell, a B-Cam operator, watched the action unfold from a monitor on his dolly. Hutchins stood over his shoulder, flanked by Souza.
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During the scene, Baldwin’s character was supposed to fast-draw his weapon and shoot at a rival. Halls had not pulled the gun’s trigger during the run-throughs he performed.
But when Baldwin entered the church to do a quick rehearsal, he apparently did.
The bullet barely missed Russell before hitting the DP and the director. The trio was about two feet from the muzzle of the weapon.
(...)
It's a long, very detailed article. Lots more at link
The day Alec Baldwin shot Halyna Hutchins and Joel Souza