Just occurred to me that maybe there WERE live rounds about the place, if, say, they had to film someone firing at something and that something shattering in a spectacular manner. Also, why were there 3 guns on the table? And another also, the fact that the assistant editor said "cold gun" (strange expression!) to Baldwin as he handed it to him suggests that there were also "hot" guns around in the studio, for whatever purpose. If these hypotheses are correct, then it would have been an accident just waiting to happen.