My understanding is that the prop guy, known in the biz as an armorer, is a girl who "should" have known her shit.
I think when all the literal smoke clears it will come out that somebody on the set irresponsibly took these prop guns out for target/shooting practice. Probably by somebody who was not raised around guns. They took a bunch of weapons, whatever was available, went out into the New Mexico desert, shot some shit up for fun, and shot so many guns that these were simply loaded and forgot about. Most likely under some kind of influence (alcohol/drugs/both). Guns were returned to the set, nobody knew they were gone, armorer got lazy and didn't check them all, and then disaster strikes.
I will also say that invariably you have to have people "in the line of fire" when filming a movie, and you may find this part controversial, but the woman who was killed bears some responsibility in this thing as well because she placed herself in the line of fire. Two people were shot, but how many people were really needed in the line of fire? Maybe only one, the camera man, and as I stated there should always be some kind of bullet proof enclosure protecting the people from the hazard.
This is safety 101. You NEVER aim a gun at somebody unless you intend to kill that person. Doesn't matter if it's loaded or not loaded. Doesn't matter that it's been commonplace in Hollywood for 50+ years (100 years ?).
In my line of work (big chemical industry/plant type stuff) you always plan the work to never have yourself in the line of fire. We literally call it that, and it certainly comes from weapon terms. I have no doubt that Hollywood in general has probably been very lax about safety on the set, especially when it comes to weapons.
