The armorer is off scot free. She had no chain of custody from the time she put the gun on the cart until AB fired it. UNLESS she was the one using the gun for target practice and the live ammo can be traced to her.
I wouldn't say 'scott free'. Most likely, it is in the job description of the Armorer to make sure no one interferes with the guns, period. And most especially after she has prepared them.
For example, lets say she prepares them 3 days ahead of time. All sorts of things happen to them. The AD grabs one to use for rehearsal, but this time checks it and finds it's messed up. That means he's going to have to stop the rehearsal, try to find the Armorer, and get her to prepare the gun properly. This is a mammoth waste of time and resources.
So I would assume an Armorer would have them locked up, or be watching and rechecking them frequently enough to notice if one had gone missing.
IMO, the AD should only be expected to do a cursory double check, with the assumption that the Amorer has done their job properly. The AD has too many other duties to be trying to get all the crew members to do their job.
But I agree, if they were being essentially stolen for private use, those are the people who broke the law.