The crew used the gun to shoot cans in the desert. I should say, some of the crew.
This underlines the lack of professionalism on that set. Live rounds are banned on all movie sets for exactly this reason. Yes, the armorer was was new (only her second job). Yes, Alec Baldwin should have double checked the gun when it was handed to him, which is also STANDARD safety protocol on EVERY movie set.
The Armorer and the production team are responsible for ALL ASPECTS OF SAFETY ON A SET. Hollywood has a long list of safety standards to avoid this kind of thing. So many things can go wrong even with blanks, they can misfire and explode in the chamber, and that's a bad day. Blanks have enough force to injure and even kill at extreme short range. John Erik-Hexum died when he pressed his blank-loaded gun to his head and pulled the trigger. The US Military, and armies around the world inset blank-adaptors into the barrels of their rifles to keep everyone safe.
This accident happened because the gun in question was not secured in between set-ups. It should have been kept in a safe until needed. It wasn't and it had been used for live target practice. It is really as simple and depressing as that.