So after doing some quick Googling, it appears that I was mistaken in my assumption about the movie biz using Prop-Only guns (given how much money is in the biz, I would've thought there'd be a booming business in modding real firearms specifically so they could only fire blanks).I think the problem occurs when you have a firearm with a barrel obstruction loaded with blanks. You could double check it is clear and running blanks but unless you put a light down the barrel to confirm the entire bore is clear you could still launch something from it. I can recall hearing about a serious injury (it may have been a death) from a reenactor firing a musket with an over charge but no bullet and part of the gun itself became the projectile when it came apart. I can imagine a scenario where a fake gun running blanks could kill someone for similar reasons. I'd bet there wasn't a real bullet involved in this accident, just people who are ignorant on how a firearm works.
Think on this for a moment, there are a ton of people who think the entire cartridge leaves the end of the barrel, not just the bullet. A lot of people simply don't know how it works.
That's actually kinda weird to me, considering how they refuse to hand actors anything that's actually sharp (mentioned it before, by I'm always amused when an actor holds up a knife/sword/machete etc. and the light glinting off the edge makes it obvious that it's more rounded than a butter knife

As a propmaster in charge of the firearms, I would think that it should be SOP to check the barrels for obstructions every single time a prop gun is checked out for the day and/or especially before use in a particular scene (as well as ensuring there is NO live ammo with the props).