Weapons Safety for Film Expert Says Live Ammo Differs From Dummy Round

There's some key differences to look for when checking a gun for live ammunition or dummy rounds ... and it sounds like it should be pretty hard to screw up ... at least in theory.

Steve Wolf, who has been working in SFX and props in films for several decades, joined us on "TMZ Live" Wednesday and literally showed us what the armorer on the "Rust" set should have been looking for.

As Steve explains, it's pretty simple to spot the difference with the naked eye ... dummy rounds have a dimpled primer on the bottom, something live ammunition does not.

Steve says there's another way to check what is going into a gun ... shaking and listening for sounds.

He says the dummy rounds are loaded with BBs and the live rounds are filled with gunpowder ... and the dummy rounds make a distinct noise when shaken and the live ammo is basically silent.

The distinctions are important ... there was a fatal shooting on the "Rust" set ... and now lawyers for the armorer, Hannah Gutierrez-Reed, are suggesting someone placed a live round in a box of dummy rounds ... and that live round bullet killed Halyna Hutchins.

Hannah's lawyers said she spun the revolver before the fatal shooting ... but it sounds like Steve is saying she should have noticed the live round, based on how the bullet looks from the bottom.

The armorer also claims she didn't know there were live rounds on the set ... despite what we first told you ... the gun from the shooting was allegedly used for off-set target practice with live rounds.