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Not the first time this theory was floated.

I thought they were doing target practice in the afternoon.

It was floated immediately after the news broke of the shooting and the disgruntled union workers walking off the set.

I just looked up "dummy rounds" and found "J&M Spec. LLC" sells dummy rounds using these four approaches for quick visual detection and safety:

	Empty Primer Pockets – This option is the most basic form of any new brass. There is an empty space where the primer (round piece in the center at the bottom of the case) would be. This option aids in identifying these as inert and is fine for most displays, reenactors, or dry-firing, though we have had trouble with them cycling in some Glock pistols after releasing the striker.
	Rubber Inserts – This is the most popular option for gunsmiths and those doing dry-fire training and function checking. The orange rubber insert replaces the primer and enables the user to see firing pin engagement as well as aiding in identification as inert. It also gives comfort to those who believe in “cushioning the blow” to the firing pin while dry-firing.
	Spent Primers In – This is the best option for film props, reenactors, and displays where the most realistic looking cartridges are required. A spent (fired) primer is inserted into the cartridge case so that the only difference in appearance between it and a live round is the indention in the primer caused by the firing pin. These are the most realistic looking dummy rounds available!
	Holes Drilled (& Empty Primer Pockets or Rubber Inserts) – For this option, we drill a single hole in each cartridge case. This is a further aid in identifying the cartridges as dummy rounds

It is obviously the responsibility of the armorer to verify that every round in the box and in the weapon is a dummy round.

And if it happened, it was under her nose, and she failed in her job to maintain a chain of custody. Not a great defense.