So while we don’t really know what happened, if anyone failed here it would be the weapons master assigned to the set: prop guns are used all the time in every movie, and are quite often discharged when people are clustered all around off screen, as the crew of the film. It is entirely the responsibility of the weapons master to ensure that weapons are safe and loaded with the proper charge, there are about 10 different kinds of blanks some of which are still dangerous up to 15 or so feet.