I am a union stagehand (IATSE Local 720) and while I have never been an armorer for a movie, I have been on prop crews and I do have some working knowledge of these things.
A lot of people are saying that the actor should check the gun before doing anything with it, but you are making some assumptions that don't always work in certain special circumstances; a movie set being one of them. This is not your local shooting range or your favorite spot in the boonies. This isn't your home. In all of these places, you are dealing with live ammunition, meaning an actual cartridge with primer, powder and bullet. If you put said live ammo in a gun and pull the trigger, you expect it to send a bullet down range. The 4 rules were written with this situation in mind. Very few people have blanks kicking around or simunitions or inert rounds vs actual live ammo. You conduct yourself in a very particular way making sure that the muzzle doesn't flag anyone and is always pointed in a safe direction. You know that when you step away from the firing line with your weapon, it should have been checked at least once and if possible, left open to verify to others that it is rendered incapable of firing (open bolt on a rifle, slide locked back on a pistol, etc) without deliberate action. YOU are the one responsible for the condition of your weapon an its safe operation at all times.
A movie set is a different animal. An actor may or may not have any practical firearms experience. The crew may have very little experience with firearms. This is why a studio will hire an armorer. They need a gun scene, so they do the responsible thing and hire an expert whose sole job is to handle the firearms and ammunition. When it comes to guns on set, the armorer is God and his/her word is gospel. It is not practical to teach and train every actor the ins and outs of gun safety. The last thing an armorer wants is an untrained person popping their gun open and dicking around with a bumbling safety check.
Could you imagine a scene from one of the Avengers movies where a crowd of 20 special ops guys have AR15s and pistols? OK, now you realize that the possibility for actors to flag themselves and others during a chaotic scene is immense. The possibility for accidents is very real. This is why there are likely no real guns in such a scene. They would all have props. Props are handled by the property (prop) master and would include fake guns. Real guns, even blank firing replicas would be handled solely by the armorer and assistant armorers.
Why do I lay out the difference here? Because the armorer is tasked with safely handling the few main actors in the scene who might actually have real guns loaded with live ammo.
LIVE AMMO YOU SAY!!! Yes. Live ammo means something different on a set. A blank or simunition is live ammo. A gun loaded with said live ammo would be called a hot gun. If I am an armorer and I have to hand an actor a hot gun, the last thing I want this untrained person to do, is begin opening the slide or cylinder and poking around trying to verify for themselves the condition of the gun. This is why I was hired, to handle the guns. The actor is hired to stand there in a fedora and pinstripe suit, deliver a line about gangster stuff and pull the trigger on an old fashioned looking six-gun delivering the execution-style shot to the forehead of the snitch.
Of course, recoil impulse can be hard to fake. Actual muzzle flash and smoke up close can be added in post-production, but the real thing looks better. So how do I as the armorer handle this? By loading a smokey blank in a real gun, verifying that everything is ready to go and when it's time to roll, declaring HOT GUN and handing the gun to the actor. The actor delivers the line, aims at the camera and pulls the trigger one time, discharging the sole blank in the gun. The other 5 "rounds" in the gun are actually inert and are just there so you don't see daylight through the cylinder bores.
Director cuts and I retrieve the gun from the actor, clear it and declare COLD GUN. Then, and only then, after I declare cold gun and have secured the firearm from the actor, will the crew begin changing the scene for another shot. The entire time the actor had the gun, there was nobody around the field of fire. The camera would have been the only thing in the field of fire at the time, and I as the armorer would have seen to that. This is why you hire an actual armorer, because actor's jobs are to have gunplay. They literally have to play make-believe with guns. If we can get away with not using real guns or using inactivated guns (the armorer will still check these) that cannot be loaded, then we will. It saves money and time if a rubber gun will work.
But no studio is going to entrust an actor to handle the safety of a firearm on set any more than you would trust an actor to make a structural weld on a building, even though they might have played a welder on TV once. You hire a professional and turn the reins over to them. You don't load a magazine full of blanks, hand the firearm to the actor and give them a 2 day crash course in gun handling. You as the armorer handle the gun and ammo in such a manner as to make it nigh impossible for the actor to do anything to actually put someone in danger.
So, in summation, movie make-believe is different from real life. The way guns are handled is different. Gun safety, while crucial, is handled differently. Live ammo means anything with powder in it. Real ammo like we'd buy in a store, should NEVER EVER EVER be brought to a set. There are different ways of making inert cartridges that cannot fire. One is simply to buy empty brass, don't prime it, don't add powder and seat a bullet in it. You can even drop a BB in the case so that a quick shake will yield an audible rattle so you know you've got an inert round. The other is repeat the process but instead of a BB in the case, you drill a series of small holes in the case, indicating that it can't be fired. The second one is good if you don't want a rattling gun in your scene. Any armorer who allows real ammunition on their set is negligent. Anyone who sneaks real ammo near a set is also negligent. You can go have real target practice at real ranges with real range officers and never have to put cast or crew in danger.
I'd be happy to answer any questions you might have.
-edit- Here are the rules and some guidelines for handling guns on a set. They are fairly expansive but you'll also find the 4 Rules among them.
https://www.safetyontheset.com/resources/amptp-bulletins/