Yes, it is indeed a sad tragedy that many people on the internet like to pretend they're experts and make "statements of fact" regarding subjects that they clearly know nothing about.
This was a professional movie set, not a gun range. The actor was given a prop gun, which he was told was "cold," and directed to act with it, which is an actor's job. So all your gun range safety pontificating means literally nothing in this context.
It is also a sad tragedy that some people like to use real tragedies, even ones where people died, in order to troll politics, or grind political axes. Shit like that really continues to demonstrate how fucking sick and twisted those people are.
First off, prop guns are real guns. Prop doesn't mean FAKE, it means property. It is a piece of property to enhance the movie scene. They use an armorer on all these sets because the guns are REAL because fake guns look bad in movies. Except when using a space gun or something else silly. Then fake guns are fine. Because they use real guns, all actors are required to take a gun safety course even if the ammo used is not normal live ammo.
Speaking of ammo, it comes in a variety of types for movie guns. Blanks, dummies, and modified rounds. Blanks are basically pop caps. These are used in non automatic o simulate a fired shot. They are a shell casing with a little bit of gunpowder in the casing and a thin film to keep the powder in. Just like a toy pop cap gun, there is no projectile because it is not needed for the gun to operate.
Dummies are basically completely rubber ammo. There is no powder. There is no bullet. These rounds are mainly used for rehearsals. They are used because dry firing a live gun on an empty breach can break the firing pin in some guns. This is a readily available product like Snapcaps which are great for dry fire training.
Lastly there are modified rounds. Due to common usage, these are also called blanks, but I am specifically calling them modified rounds here to avoid confusion. Modified rounds are for any form of automatic firearm requires a projectile to operate. This is because the firearm uses the force from the gas that is pushed back from the projectile to move the reciprocating part of the gun that resets the firing function. In most handguns this is the slide. It could be a bolt in a rifle for example. If a blank round is used in gun that requires a projectile to reset, the gun will not cycle correctly. Since movies don't want lethal rounds, the modified rounds use a WAD instead of a bullet as a projectile. The wad is something that is made of paper, cotton, wax, or whatever soft material that typically disintegrates shorting after being fired from the gun. Because the modified ammo is shooting a wad, it also has to have far less powder to shoot that wad. However, the issue is that with far less powder, there is not enough force from the gas to cycle a normal gun designed around normal live ammo. So any form of automatic gun used in a movie has to be modified by using far lighter springs and reciprocating action. Which also means that if a live round was put inside an automatic action gun, the gun probably would be damaged severely if fired. The other thing to note is while modified rounds are certainly less lethal, they can still be lethal if the barrel is put up against a person before firing. This is because a projectile, the wad, does shoot out and can penetrate a body with enough force to kill a person for a short distance after fired.. There has been incident where this has happened before.
The movie Rust was a western and they were using revolvers. Specifically a Colt revolver. Since revolvers aren't automatic action guns, they can use blanks instead of modified rounds. As such, revolvers are not modified typically in anyway for movies. Especially actual antiques actually used in the movies for period pieces. This is also why it is required for anyone who will be handling the guns to have gone through a gun safety course. Specifically Alec Baldwin has done several throughout his career as it has been mentioned in the media already. So Alec should know the basics of gun safety which is to ALWAYS CHECK A GUN FIRST even if handed to you by a person saying it is unloaded.
Lastly, revolvers can fire modified rounds or true blanks. The problem is that modified rounds for revolvers have to be a specific length or the projectile that comes out can damage the gun.
As far as my gun safety pontificating... that is the law and ignorance of it doesn't usually protect one from it. Money and politics might though. Just for reference similar incidents like this have occurred outside movie sets. Been times when some person slipped a live round into a gun at a gun store. Employee grabs gun to hand to the next customer to check out. They don't look that the gun is actually empty but assume it is because it was handed to them from a store case. They fire and bam! Criminal negligence charges happen after at the very least. This is even for people with no training at all.