Yeah, I agree you.
The armorer loaded the gun. She should be able to tell if a round is real or a dummy.
The question not answered yet is how live rounds got into the box of dummy rounds.
As somebody posted above, she did not load the gun. She wasn't even on the set that day. She was actually only the 'part-time' armorer since she had been required to do other jobs like Property Master and was off working on another set that day.
The Assistant Director handed Baldwin the gun and it's assumed that he loaded it or at least checked it.