Prop gun was used for target practice off set

Dummy rounds available on Amazon

I wondered how this could have happened. 

TMZ is reporting that the prop gun which killed DP Helyna Hutchins and wounded director Joel Souza had been used by members of the crew for target practice away from the set. 

And they found live ammunition stored alongside blank ammunition on the set.

If you're rushed and a bit sloppy in your work, can you mistake live rounds for dummy rounds? Did they use dummy rounds? Was the gun supposed to be loaded with dummy rounds? 

I keep thinking back to that jackass film prof who was incensed that college students would use anything but real guns in a student film.