The Variety article on the closing arguments is fairly succinct.
The prosecution has argued that the evidence shows Gutierrez Reed inadvertently brought them to set, mingled among dummy rounds. In a police interview, Gutierrez Reed said she brought some dummies that were loose in a bag in her car and were left over from her previous job as armorer on “The Old Way,” a Nicolas Cage film.
“I’m not telling you that Hannah Gutierrez intended to bring live rounds on set,” Morrissey said. “I’m telling you that she was negligent. She was thoughtless. She was careless… For all we know those dummy rounds were floating around the set of ‘The Old Way,’ and Nicolas Cage is lucky to have walked away with his life.”
Throw in the fact that she apparently didn’t even give the rounds a little shake to confirm that they were empty except for the little BB rattling around in there, and “without due caution and circumspection” is a slam dunk. Baldwin’s case is a little more iffy, but certainly a reasonable one to bring to a jury.