The rumor in Hollywood is that the armorer took the guns out for target shooting during a break.
*IF* that's the case, that's criminal-level insanity.
Not if the weapons are cleared at the end of the range shoot and the ammunition is stored/handled properly.
If a senior actor and producer tells the (inexperienced and young) prop gun wrangler, "it's fine, I know what I'm doing, I checked it", though...
Filming a drama involving weapons, you know these props are going to be pointed at people and the trigger pulled. Checking and rechecking seems reasonable to me.
Lot of blame to go around here.