I’m basing what I say on the Wikipedia article and the two years old Legal Eagle video, but it seems it wasn’t his negligence, but rather the armorer’s and the assistant producer’s:

According to a search warrant, the guns were briefly checked by armorer Gutierrez-Reed, before assistant director Halls took the Pietta revolver from the prop cart and handed it to Baldwin.[39][40] In a subsequent affidavit, Halls said the safety protocol regarding this firearm was such that Halls would open the loading gate of the revolver and rotate the cylinder to expose the chambers so he could inspect them himself. According to the affidavit, Halls said he did not check all cylinder chambers, but he recalled seeing three rounds in the cylinder at the time. (After the shooting, Halls said in the affidavit, Gutierrez-Reed retrieved the weapon and opened it, and Halls said that he saw four rounds which were plainly blanks, and one which could have been the remaining shell of a discharged live round.)[41] In the warrant, it is further stated that Halls announced the term “cold gun”, meaning that it was empty.[39] Halls’s lawyer, Lisa Torraco, later sought to assert that he did not take the gun off the cart and hand it to Baldwin as reported, but when pressed by a reporter to be clear, she refused to repeat that assertion

Btw, holy fucking shit I hate lawyers