I saw that mentioned on another site. I had never heard or read that prior to this incident. How does that work?
My understanding is that there are a handful of different types of guns used on set.
1. Non-firing dummy guns. These are often cast in rubber of varying durometer or plastic then painted, and can be used as stunt rifles where someone needs to be hit with one, or carried by extras, etc. Some are really nice hero pieces made from metal, but they cannot fire and can be handled without particular care. If these are "fired" in the movie, it done via CGI.
2. Guns that cycle safely but do not fire. Green-gas powered blowback airsoft type guns with no pellets are an example of this, as are some CO2 BB guns. No projectile leaves the barrel, and no gunpowder is burned. CGI is used for muzzle flash.
3. Dedicated blank firing prop guns. These use cartridges for which regular ammunition will not chamber. As mentioned, the one I had was 8mm. These can be "front firing" which means it vents to the front out the barrel (which also necessitates additional precautions and standoff distance between the actor and what they are pointing the gun at) or "top firing" which means the vents come out the top of the gun and the barrel is completely blocked off. You still have to be careful about the gasses that come out the top but your standoff distance can be much shorter.
4. As dogmush mentioned, some still use real firearms professionally converted to shoot only blanks. Not sure I'd agree that they are the most convincing on film - they'll still typically use CGI to add muzzle flashes because directors prefer the drama of huge fireballs over what a blank usually produces. Nor are they the cheapest option (certainly not per round fired), but they can be more convincing to the actors. These can be converted a number of different ways, but if they are semi-auto they have to have a barrel restriction (often welded in) to cycle, and often have locking lugs ground down and so forth. These can be the most dangerous option as depending on how they are converted they may still chamber live rounds, and depending on how the muzzle restriction is installed that could come flying out as a projectile.