I believe there is something rotten.
No deadly cartridges should have been available for use on the set as far as I know.
How did the deadly cartridge get there?
Generally speaking, that is correct.
How did the deadly cartridge get there?
Producer Alec Baldwin overrode the Armorer's and Safety personnel authority and orders.
Baldwin allowed set personnel and outsiders to use, and practice with, live ammunition in their off hours on the set.
In effect, Baldwin allowed people to Play With Guns on the set.