On a movie set, where you could have lots of actors shooting at lots of other actors - sometimes both ways - and you may have several retakes to get it right, it probably is not feasible to get every actor to check every gun and every round in every mag. They will have to rely on some specialist to check instead. I am not saying it is the best practice, just saying these are practical concerns.
Also, not every actor who acts with a gun is necessarily a firearm owner and user. if there was a live round in a gun by mistake, it could be just as dangerous to rely on someone who do not what they are doing to identify and remedy the situation.
It is for this reason when you fly Jetstar, they do not let you inspect the plane. You rely on some aviation technical whom you do not know and never met to make sure the plane is airworthy.