How do you check a round to see if it's live or a blank?
Normally, like this:

But with revolvers, you can see the bullets in the chamber, so where the camera would see that, you use dummy rounds instead of blanks - blanks have no tips (but do have propellant), dummy rounds have tips (but no propellant).
So, in the case of dummy rounds, they pretty much look just like a live round, except their primer (the base of the bullet) is dimpled, and they make a rattling sound when you shake them.
All of this leads us back to the armorer, who should have ensured that the round in the gun was either not in the gun (if it wasn't called for in that scene) or that it was a dummy round (by checking the dimple and the rattle).
My sidelines guess is that she looked at the box she'd been given that said "dummy rounds" on the side and figured it was all good to go.
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Baldwin's now saying he didn't pull the trigger, either - that the gun just fired.
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I agree with X that things got a bit fierce towards Shark and there was a bit of a pile on. Sorry, Shark.