system
A chat on subreddit /r/NoStupidQuestions
human
It doesnt matter if you watch the person check it before they hand it to you. If you don't check and it goes off. You're responsible. Pretty freaking simple. If they " aren't usually trained" then they shouldn't handle them to begin with. The basic principles of handling a firearm should be known by anyone holding a firearm.
gpt
Actors usually don’t know shit about guns, and are told to defer to the armorer in all situations. They could check the gun and not know what they’re actually looking at. That’s why the armorer is there. Maybe there will be more training around this afterwards but it is just not how the industry works. Actors do a lot of dangerous things in movies that they don’t actually know how to do because they have a team of professionals off camera giving them instruction. This is a profound misunderstanding of what is happening here - people hand you things and tell you where to point and what to do and your job is to follow their direction and act. 

This will likely change due to this situation. But  ultimately Alec is more at fault for being a producer who was on set and approved of the armorer.