Sure, if there were any other deaths due to this kind of mishap, we'd know. But what we don't know is how many close calls there have been, how many times people were lucky and got away with it. Those don't make the news. Maybe there have been a lot. Maybe it practically never happens. We don't know, at least yet.
We also don't know the level of chalance or nonchalance, due to cultural influences, that are attended to this kind of job, at least in America as compared to how it is handled in other countries. Or, for a more local example, versus how other potentially lethal props, such as explosives or wild animals, are handled. I think it's possible armorers might be perhaps marginally looser with guns than an explosives expert is with explosives, or a wild animal expert is with wild animals, specifically because of guns' ubiquity and our uniquely American cultural attitude towards them as being NBD. I'm not saying I firmly and incontrovertibly believe this to be the case—only that I think it's possible, and something I can't immediately dismiss out-of-hand as impossibly ridiculous simply because I myself certainly wouldn't treat it that way.
In any event, it does seem to be increasingly clear that both the assistant director and the armorer on this particular movie treated the use of prop guns on the set with a certain degree of nonchalance.