Apparently there is a safety protocol on movie sets when using guns replica or real. From what I gather at least three people are in place to check the "prop gun" before it gets handed to Baldwin. So there can be an argument that he presumed its safe to "fire". Nevertheless, again from what I read, he too is required to check to see if there are no projectiles in the barrel. Another reason that has been repeated is that this was a low budget movie with only 21 days to shoot. Maybe everything was being rushed the get the job done on time. There is also stuff in print today about cost cutting and not having sufficient number of people on set whose principal job is gun safety-like doubling up responsibilities and hiring those with limited or no experience handling weapons.

This will all come out during law enforcement investigation and of course during "discovery" in the expected civil case(s).