I think I see the distinction this guy is trying to make now. “Dummies” are non-firing cartridges that are made to look real. They are not blanks in any sense. I don’t think he made that clear. “Dummies” would be inert “blanks” would not. Perhaps “dummies” do have BBs in them for the purpose of making them identifiable.
Still, why would there ever be any live rounds on any movie set? They would have no legitimate purpose.
Yes, this is correct, and you're also dead on about live rounds on a set. That's a recipe for disaster. Add in a careless and inexperienced armorer, a producer who ignores repeated firearm mishaps, an assistant director who was fired from a previous movie for a firearm mishap, and a general disregard for firearm safety.
Joe Swanson—whose company, Motion Picture Blanks, in Kingman, Ariz., has manufactured movie ammunition for about 35 years—says he supplied the “Rust” production with blanks as well as dummy rounds. Dummy rounds contain no explosive ingredients and are used as stand-ins for real bullets on camera.While blanks typically have a crimped tip that distinguishes them from live ammunition, dummy rounds have a nearly identical look and heft of real bullets.
As a safety measure, Mr. Swanson says he inserts a pellet inside each dummy round so that it will rattle. “You can’t really tell the difference unless you shake it,” he says, adding that it’s typically the job of the on-set firearms safety coordinator and assistant director to do such safety checks.
There is a lot of conflicting information out there, and it's possible there was other dummy ammo used in addition to that described above, which would add another tent to the Rust circus.