I’m guessing the camera director and film director were standing behind a camera to line it up and see what it looked like when Alec fired towards the camera. Someone slipped a live round into the pistol. Best guess.
Another thing to consider too is that if the movie was striving for realism, they would have been looking at a cylinder full of lead nosed bullets too. A guy in Baldwin’s position could have even done a quick spin of the cylinder and missed the one with a live primer in it.
