Real guns are cheaper than prop guns on set, so they used a real cowboy revolver. The gun had to fire for a scene, and since it was a revolver there needed to be real bullets (the camera would pick up blanks as revolvers have an open end cylinder that is really obvious on camera). The prop gun person in charge was a nepotism hire (daughter of an old movie prop pro) with little experience. And Baldwin as a nogunz retard probably was flagging the crew like crazy with the pistol and had an negligent discharge as a result and killed a crew member.