Well, it was a prop (property) on the set. The “plan” that the director, camera operator, and Baldwin were following was that the gun would be loaded with a wad so the camera would get the shot of the flash from the muzzle.

Lots of stupidity here: who wrote the scene that way or wanted THAT angle. Even with that angle, why were the 2 people standing down barrel from an even blank loaded gun—why couldn’t the camera be operated remotely? Perhaps the focus needed to switch from Baldwin to the end of the barrel—but why couldn’t that be done remotely? Where did the load come from? Who handed the person who loaded the gun that specific round—the live one? The prop master? Who was visiting the set that day? Who was back in the prop area not assigned to the film that day? That’s your guilty party. Not that Baldwin’s a nice guy, but he more than likely was surprised as everyone else that an actual round came out of the gun.