The current story, from a union local's email, is that the prop master accidentally gave Baldwin a regular gun loaded with a live round. In that possible scenario, Baldwin didn't know he had a live round, fired "into" (near) the camera, the bullet went through the woman who died, and wounded the second person, who was behind her. At close range just about anything larger caliber than a .22 LR or .25 ACP would do that (and maybe those calibers would as well).
Knowing how the "news" works, it'll probably be a day or two before what happened is reasonably known. One possibility that would fit with the above would be that the prop master had two similar prop guns prepped for two scenes, one loaded with blank(s), one with live round(s), and he gave Baldwin the wrong prop gun for the scene being shot.