This Dan Abrams video has an expert film armorer named Clay Van Sickle demonstrate examples of blank, dummy, and live ammunition.
I noted that the dummy round used in films does not have a hole in the side. It has the primer punched to deactivate it and BB's inside that can be heard if shaken. It visually looks like a live round except for the punched primer which can be seen from rear of the guns cylinder when the loading gate is opened and the cylinder spun.
I think AD Halls meant to say "rear" instead of "side" when talking to LE. That would make much more sense. JMO.
Timestamp 10:56.